You're Doing It Wrong is an achievement introduced with 5.4's Proving Grounds, where you have to complete the Silver challenge while in a role not suited for it. I've been pretty pumped to try this, since I already sometimes tank heroic dungeons in Holy spec/gear just for fun (and loot satchels) anyway!
This was very challenging. It took me many tries, with different talent/glyph setups, and a variety of tricks. Gear doesn't help, since everything is scaled down, so it's pure skill here. Well, and luck.
Here are my observations and tips for others looking to try this!
How does a healer tank, without heavy armor, defensive stats and defensive abilities? Easy. We just heal through all the damage we take.
One nice perk about being a paladin is that we won't really have to worry about threat, due to Righteous Fury. We can generally hit a mob just once with a Judgment or Holy Shock, and they'll be locked onto us pretty good. And of course, we also have Reckoning.
Having said that, we don't necessarily want to tank all the mobs. It's okay to let Sikari tank a few, since we can heal her. Having all the enemies on either yourself or Sikari can quickly end in death/failure, but if you split them up and divide the damage, it can be a lot easier to maintain comfortable health levels.
Finally, we need to contribute some decent DPS, if we don't want to fall behind on the waves. This means heavy Denounce and offensive Holy Shock usage, as well as offensive Word of Glory (if using Glyph of Harsh Words).
For the most part, gear doesn't matter in Proving Grounds - everything gets scaled down to 463. That being said, there's a few basic things to remember. Gear doesn't get scaled up, so if you have pieces below 463, you'll be at quite a disadvantage. Next, gem sockets and their bonuses DO get retained after scaling, so the more sockets, the better. Yes, this means that a belt with two sockets from Heart of Fear LFR will be better than a Throne of Thunder belt with no sockets.
As far as secondary stats, I recommend focusing on Haste/Crit. I found the Mastery shields too small to be truly effective, given how much time you spend DPSing and not healing. And Haste and Crit both benefit your offensive attacks, while Mastery does not.
Also remember that Legendary cloak/meta procs and Sha-touched gems do not work. If you have a spare helm (and gold to burn) you might want to use a Burning Primal Diamond instead. And if you are using a weapon with a Sha-touched gem, any weapon with a regular socket instead will be more effective.
Finally, eat your buff food! I used +300 Intellect food and Crystal of Insanity. I used this for +500 to Intellect, Spirit and Stamina, but obviously a pure Intellect flask is fine too.
Your talent choices are extremely important if you want to beat this. Let's take a look at your options.
Tier 1: Speed of Light, Long Arm of the Law, Pursuit of Justice
Any of your speed talents are decent, though I'd personally avoid Speed of Light - you never have to run THAT far in Proving Grounds, and the long cooldown is annoying. Long Arm also gains extra value if you're Judgment kiting, but more on that later.
Fabulor's Choice: Long Arm of the Law.
Tier 2: Fist of Justice, Repentance, Evil is a Point of View
Tough choices here. Fist of Justice is very handy on the later waves, as some of those mobs deal a lot of damage. Being able to stun them more often and from range is very helpful defensively. However, I found removing an enemy from the fight entirely with Repentance (which works on every monster you will face in this challenge) was ultimately more beneficial. Evil is a Point of View would work too, I just prefer not to have the incapacitated enemy running around all over.
Fabulor's Choice: Repentance
Tier 3: Selfless Healer, Eternal Flame, Sacred Shield
Selfless Healer or Sacred Shield. You will be judging a lot (for threat), so Selfless Healer will get a lot of mileage, but Sacred Shield is nice to have as a constant passive damage buffer, and it's low-maintenance, since you simply refresh it every 30 seconds (or earlier, if you have a free GCD). You can also shield both you and Sikari, which comes in pretty handy.
Don't take Eternal Flame. It no longer builds Mastery shields, there's only two people to heal, and the HoT portion will often go to waste since the damage you will be taking is rapid and spiky, not slow and steady. Perhaps most importantly, though, is that taking Eternal Flame means you cannot take Glyph of Harsh Words, limiting your DPS options considerably.
Fabulor's Choice: Sacred Shield
Tier 4: Hand of Purity, Unbreakable Spirit, Clemency
One of our major weaknesses in this challenge is that unlike other healers, we cannot cast our main tank mitigation cooldown (Hand of Sacrifice) on ourselves. Hand of Purity gives us a small but helpful mitigation ability on a short cooldown, which helps. More importantly, it lets us negate the Chomp bleed debuff applied by Illusionary Foragers, which is one of the more dangerous elements of the entire challenge for us. That being said, if you've taken Glyph of Divine Protection, Unbreakable Spirit becomes a powerful option too, giving you 20% physical mitigation every 30 seconds.
I have Clemency in the video, but this was a poor choice. I'd avoid it. You will be doing most of the tanking, and you can't Hand of Sacrifice yourself, so being able to cast it twice is...not that useful. Clemency CAN be used effectively to bounce mobs between you and Sikari with liberal use of Hand of Protection/Hand of Salvation, though - more on this later.
Fabulor's Choice: Unbreakable Spirit (if taking Glyph of Divine Protection),
otherwise Hand of Purity
Tier 5: Holy Avenger, Sanctified Wrath, Divine Purpose
All good options this tier. Holy Avenger and Sanctified Wrath will let you dish out more healing/damage in the later waves when you really need it, while Divine Purpose will provide you solid benefits the entire time. Personal choice, really. I struggled in the later waves, so I found the increased damage from Holy Avenger very helpful. (Be sure to Crusader Strike when Holy Shock is on cooldown for additional buffed Holy Power generation.)
Fabulor's Choice: Holy Avenger
Tier 6: Holy Prism, Light's Hammer, Execution Sentence
As is usually the case for Holy Paladins, Execution Sentence is garbage here. You might think the huge single-target heal would be really nice for tanking, and it's certainly not bad. However, the other talents far outshine it for one main reason: AOE threat. We completely lack any baseline AoE attacks, so having Holy Prism or Light's Hammer available to round up a large number of mobs at once is invaluable. And their damage/healing is quite nice, too.
Which one is better? Once again, it comes down to personal preference. I find the long cooldown on Light's Hammer difficult to work with, though it does let you almost completely negate the troublesome waves of Illusionary Foragers. Holy Prism lets me establish threat on as many mobs at once as Light's Hammer, and I can use it far more often. Either can work perfectly fine for this challenge though.
Fabulor's Choice: Holy Prism
Glyph of Harsh Words - Lets you use Word of Glory offensively. We don't have many offensive abilities, so gaining another one is incredibly useful. Recommended.
Glyph of Denounce - Straight-up increased DPS output. Important glyph. Recommended.
Glyph of Divine Protection - The majority of the damage we will be taking is physical, so being able to mitigate 20% of it is awesome. Recommended.
Glyph of Holy Shock - Decreased healing, increased damage. Definitely a bit of a risk, but if you're focusing on burning the mobs down quickly, this will probably be a mandatory pick.
Glyph of Word of Glory - Increased damage sounds nice, but remember that a large chunk of our Word of Glories will be used offensively, negating the benefit of this glyph.
Glyph of Avenging Wrath - Free healing while your wings are out isn't bad, but your other glyph choices are probably more useful.
Glyph of Burden of Guilt - See tricks, below.
The most valuable trick to this challenge is simple experience, honestly. Knowing which mobs are dangerous, which need to be killed ASAP, and generally just learning the flow and rhythm of how much healing versus how much damage you have to be dealing. However, here's some specific tips that might help.
• Judgment Kiting - If you take Long Arm of the Law and Glyph of Burden of Guilt, you can kite mobs fairly effectively. However, the tight time limit made this mostly unfeasible for me. Still, it's got possibilities.
• Bouncing mobs - You can make monsters waste a lot of time by forcing them to run back and forth between you and Sikari, while the two of you continue to DPS them. If Sikari is tanking the mobs, you can make them all come to you by casting Hand of Salvation or Hand of Protection on her. Similarly, if you have threat, you can cast either of those on yourself or Divine Shield to send them all at Sikari. Being able to cast both Hands twice with Clemency obviously makes this tactic much easier.
• Foragers (Vermin): Their bleed HURTS if it stacks up too high. We don't have the AoE to kill them quickly, so if you try to tank them all, the stacks will ramp up and you'll be taking a lot of damage. Nullify the bleed with Hand of Purity or let Sikari tank a few on her own. If you use Light's Hammer on them, you'll have threat on all of them whether you want to or not, but the Hammer's AoE healing and damage will get you out of the wave without too much trouble.
• Flamecallers (Yaungol): They put lava on the floor. Stay out of fire, put enemies in fire. Simple, but the extra damage we get from these is really important.
• Wind-Guards (Mantid): These guys are assholes. They deal a ton of physical damage and knock you back pretty often. When they do so, they wipe your threat and head right for Sikari. You will face these mobs in wave 6, 7 and 8. You will want to use defensive cooldowns, stuns, and healthstones when tanking them. Learning how to deal with these guys was the only thing keeping me from the achievement, for quite a few attempts.
In Wave 6, I cast Repentance on the other mob (a saurok), so I could focus 100% on burning the Wind-Guard down. When he knocked me back, I let Sikari tank him for a few seconds as I healed myself up, then taunted him back. REALLY important to kill the Wind-Guard before the next wave started.
In Wave 7, there's more enemies, so I cast Repentance on the Wind-Guard this time, and dealt with him after the others were dead. Again, very important to kill the Wind-Guard before the next wave!
In Wave 8, once again Repentance on the Wind-Guard initially, deal with everything else, then take out the Wind-Guard. Things didn't work out quite as planned in my video, but you can see that the Wind-Guard didn't give me much trouble. You can take your time here, and leave the Wind-Guard CC'd for as long as you want - nothing happens when this wave "ends", so take your time!