Going to copy/paste some stuff I wrote in another thread + some extra. I'm ilvl 103 or 104 right now, T6-8 on farm, progressing T9, for reference.
A bad whm is made by:
cure2/cure3 spammers - not using enough cure1 for freecure procs
inconsistent or nonexistent regens
poor timing for heals (many times you need to precast in prep for incoming damage)
poor use of cd's
overall terrible mp management
WHM main tank healing is from regen + cure1 + freecure procs:
You're going to run out of MP quickly if you spam anything, much less cure2/medica/etc. So keep regen up and sit on damage a little bit where appropriate (remember to only recast regen when safe to do so). Youll have to learn the pace of the fight to do this effectively, but basically, you don't want to cure immediately all the time. Regen and try to time cure1 for after hits, not even every hit. requires some coordination with the other healer. Use your freecure procs and know the limitations. Freecure is a free cure2, and the freecure buff lasts a good amount of time... Wait on the tank to take more damage before you use it. freecure is good to use in place of a cure1 or cure2 depending on the situation just because it is free -- but remember, it is a cure2 so watch your aggro. It's more ok to overheal with a freecure proc than a cure 2 simply because it is free -- but try not to use it until there is a cure2 worth of damage done to the tank if you can help it. Either way it generates 100% aggro of a regular cure2.
Keep in mind that while physic+embrace is about as powerful as a cure2 at the price of a cure1, the fairy can't embrace at the same rate as the sch's physics (it is slower). WHMs also have regen which people tend to forget to include in that. Regen should be up on a tank most all the time. And don't forget you can divine seal + regen for a really nice hp regen.
WHM main raid healing comes from medica 1/ medica 2 depending on the situation:
Under some conditions, a cure3 is great as well but shouldn't be the go-to raid heal due to its cost and radius. (a med1/2 is much better in many cases if people are spread out a little more and are more likely to also hit the tank where cure3 might not). Basically, you want to try to time your med1/2 to go off soon after the damage is done. Some situations call for cure 3 like in T8 if the towers get proc'd a little off and defensive goes off while allagan field is up. You want to land a cure3 immediately after defensive hits so the party can take the allagan field damage. Things like that. On the opposite end of the spectrum, for fights like T6 -- I use divine seal+med2 before chains+mark to help keep health high during chains. If several people start getting lower, I will position myself to hit them with a med1 before spit. Then med1 after spit. Usually, I can do the one med2 and one med1 though. The remainder of the med2 will finish topping everyone off. In fights with raidwide damage where you dont expect MORE raidwide damage, a med2 is good to get a little health up and then EVENTUALLY top them off. Also remember that some hp is naturally regenerated over time. Oh yeah, and like I said, timing can really help even on raid damage, so for the raid damage that interrupts casting (not actually all that many, but fireballs in T5 is an example as well as leviX's raid damaging move where you have to hit the switch for the shield) use surecast! Keeps you from being interrupted so you can time the aoe heal to go off right as the damage comes in and move right along.
WHM have their potency cd/ spell speed cd and/or regen+cure2 for low hp tanks:
Benediction can usually only be used 1-2 times in a fight, so use it wisely. For certain moves, stoneskin is important before a hit. Otherwise, use your cd's! Divine seal is awesome to pair with regen, cure1, cure2, med1/2. Not so much cure3 since its already so potent.
WHMs manage hate through paced healing and SoS:
Like I said before, pace yourself! Just because heavy damage occurs to the raid doesn't automatically mean you should be using cure3. And you shouldnt be sitting and spamming cure1 or 2 either. take it easy. time to the enemy's attacks. Learn the fight super well. Generally, SoS will keep your hate down if you use it regularly for mp as you should. However, sometimes shit happens and you will need to use it just to lose hate, especially if you accidently use something too potent as a new mob spawns! Additionally, stoneskin is practically free as far as hate goes. Use it for things like when the dreadnaught spawns in T8. Not always necessary of course, but it can be useful to keep hate down while providing som ehp to the tank.
WHM manage MP more through timing/pacing:
sticking with regen+cure1 as much as possible except for using that freecure proc. Good management of SoS is also extremely important. When you hit between 60-80% (up to you honestly) use SoS and use on cooldown/as needed. Some whms forget to use this often for mp. Additionally, it is beneficial to get some PIE from gear. Up to you how much. I personally go for det and pie over spell speed and crit. spell speed imo is not a great stat because its usefulness is completely avoided by my playstyle since I dont spam moves (generally) and since I try to time moves. Spell speed and spamming, again, will just blow your mp away with little utility in MOST situations. + you have that spell speed buff. And you should have swiftcast (I dont save that for raises. good to have for clutch raising, but its useful for so many other things too). The benefit of timing/pacing and basically sitting doing nothing occasionally is that MP regenerates over time (and based on your PIE). Additionally, if you want more PIE you can eat pineapple juice. It's inexpensive to craft -- only takes clusters and pineapples, so right now at least, its my go-to food. Finally, I try to take some Xethers or High ethers or something for mp restoration in case of emergencies or just things going wrong like too many deaths where my mp gets depleted too quickly. I almost never actually need them, but sometimes I use them anyways for good measure ha. Otherwise, some mind potions are good for a divine seal-like effect when you dont have divine seal up.
WHM dps is somewhat lackluster for single-target:
I generally leave single target DPS to my SCH buddy while I keep at the healing. Sometimes Ill throw something extra out too but not too much. However, WHM excel at aoe damage. Holy does not have a target limit like bane does, so it works really well on very large pulls while also providing a bit of cc (particularly useful for dungeons and killing trash quickly. and stun keeps tank from taking damage for just a bit as well). However, Regen needs to be put up possibly with divine seal before-hand because benediction and stoneskin are the only moves really that arent affected by the stance change. So you have to be extra careful on the timing. Takes getting used to. Cleric's stance dancing!
Both healers need to be using virus and eye for an eye:
appropriately and without accidentally both using it at the same time, so takes some coordination there. These skills are great in many situations and should not be neglected! But be careful because SCH virus has a little more utility, so in some spots they should definitely get priority on that move.
Raising:
the general order for who does raises is SMN->SCH->WHM but it doesnt have to be that way. If a WHM is doing well on MP they can raise just as easily. My group often does it with that priority but also based on who has swiftcast up. Interestingly, if none have swiftcast up, the whm could try to use the spellspeed buff and then cast it for a faster raise.
Again, SoS is for mp management as well as hate management. Try to save it for the mp if you can!! But it can be good to blow simply for emergency hate issues. I try to save it til I need it for mp, and sometimes I will be in the yellow or orange before I get to that, but it only matters in the end if you take hate and get hit. Just watch for that.
Be wary of your cast times:
Stoneskin has a longer cast time than your cures. Things like that. Also, learn when you can start moving again without interrupting a cast. this can really help you out! Also, for stuff like plumes: dont finish casting... GET OUT OF THERE or you're probably dead. If the tank needs an emergency heal right then, use benediction/swiftcast cure1/2, or pray sch lustrates.
And most importantly -- trust the other healer!
If you can't trust the other healer, you need to find a new one. or improve yourself (as maybe the actual case is the other healer can't trust you!). Healers work best if they are really in-sync with the fight's pacing etc. and also with each other. Knowing what the other healer is going to do helps you know what you need to do, so it may take a little time getting used to each other while also getting used to the fights. But it can really work well!
There's certainly many things to add, I'm sure. Reply if anyone else thinks of more tips
