The healer is the core of the group, the one who keeps people alive. The difference between an average healer, and a great healer can and will make all the difference between winning and losing any fight and a good healer is ALWAYS welcome in any group.
General Healing Tips
Make a decision. The first and most important part of being a good healer is to make the choice. Do you want to be a healer, and only a healer, or do you want to mix offense and defence? If you choose the former, you are choosing to do everything in your power to heal, and nothing else. The latter, and you'll be somewhere in between healer and DPS. It's fairly obvious what the best healers pick.
-I have all my damaging abilities on hotbar 5 (I use 4 hot bars, the small cluster.) To do this, simply push the little arrow at the side of the hot bar and put your offensive abilities in there, meaning your 1-4 won't be cluttered with offensive abilities!
Watch everything. Some healers will say that all you do when you're healing is play ‘whack-a-mole', and heal whoevers health is lowest by clicking on them in the party window. These players have got it wrong. If you're only watching the health bars, you're not going to do nearly as well as when you watch the fighting. By casting spells on people before they engage the enemy you dramatically increase the healing potential you can do. But to know who to cast them on, you got to watch the battle!
You have to know;
1. Where everyone is
2. Where the enemies are
3. What class people are
4. How much danger they are in
From that, you have to be able to make a quick decision on what to do. For Example, you are a Rune Priest, and your friend the Knight of the Blazing Sun is getting attacked by 2 people, your friend the Slayer is getting attacked by two, and your friend the Shadow Warrior is getting chased by another. You also have a Witch Hunter on half health next to you.
In this situation, the Knight, being a tank class will take less damage than the Slayer, who is a DPS class with not much defence. The Shadow Warrior is not in as much danger as either of those two, and will receive little to no damage, provided he stays at range. The Witch Hunter isn't in danger at all.
How you approach this problem is totally up to you, however, you will have to provide a level of support for them all. The Slayer is going the need the most support, and the Witch Hunter the least.
A great mod for keeping track of health is Squared by Aiiane along with Phantom by Aiiane. In squared, set your bars to HP colour, and names to class colour. This way you can very quickly identify see who is getting hurt, and what kind of class they are.
Understand your tools. It's important to know what your tools do, so that you can use them the best way possible. Casting a HoT on someone who needs a big chunk of health now isn't good management. Neither is wasting your time with a long charge up cast on someone who is almost at full health, and just needs a top up.
-HoTs give you slow, prolonged, large health return, and are usually instant casts. Great for tanks, healing someone hurt who isn't in direct danger, and for on the run.
-Charge up heals give you a medium amount of instant health return at the end of their build up time. Great to cast on someone who is about to receive a whole lot of damage.
-Instant return heals give you a small amount of health, are instant casts and are often very AP inefficient. Great to cast on the run.
-Group heals help to keep your entire team up.
Use Your Team. You are there to help your team, and they need to be there to help you! If you need it, ask for your tank to guard you, or a DPS to dispatch anyone foolish enough to attack you! If you find yourself under attack, run into your team.
Prioritise your heals. This DOES NOT mean heal the person on the lowest health. Don't waste your time healing someone on ½ health who is perfectly safe when someone on full health is about to get in some serious trouble! If you have time to throw them a HoT, do so, but focus on those in trouble first.
Cleanse yourself mate. There are a lot of debuffs that can be cast on healers that can seriously reduce the healing that you do. An example of this is the sorcerers Word of Pain (Reduces Willpower and deals damage after 10 sec). This can be cleansed, and should be cleansed before the 10 sec timer is up!
Use your Detaunts!! Seriously one of the best things you can do. Detaunt that Choppa BEFORE he gets close enough to hit you. Detaunt that Witch hunter as soon as he appears! After you have detaunted, load yourself up with 1 or 2 HoTs, get near your friends and continue to scan the battlefield for others who need healing.
Heal over Time's (HoT's)
Fire and forget spells which slowly return health to your friends. These are a core part of healing, and make up about 50-60% of what you should be casting. They are highly Action Point (AP) efficient, and free you up for more important things. There are some things you can do to improve your game with HoTs.
Stack ‘em if you got ‘em. If one HoT is good, two is even better. Use as many hot as you have. Even if one is good e.g. ‘restores 500 health over 15 sec' and the other is worse than the first e.g. ‘restores 300 health over 20 sec', you want to stack them on someone who could/is taking a lot of damage.
Run Healer, Run! Most HoTs can be cast on the run! This means if you are running away from an enemy, you can still cast your HoTson your friends who are getting hurt!
Pre-HoTing. If you cast your HoTs on people before they start getting hurt, you'll have lots of breathing space to cast your big heals, and you can react to other things without worrying if someone had a HoT on them.
Boo, there you are! Because most HoTs are instant casts, they only require a moment of Line of Sight (LoS). This means you can peak out, cast them, and then hide again, or cast them as you run past a group in a cramped spot.
Casting Time Heals (I call them big heals hehe)
A large amount of health all at once after a cast time, these will be your staple heals for when one person is in diredanger.
Time it! If you cast these before someone takes damage, you'll hit them with it right after they take the damage(because of the cat time)!
Don't put yourself in danger with them! If a group of enemies are charging at you, cancel your spell and run! Don't get caught out. You will be set back if you are hit casting these. If you have one person on you, its generally not too bad, but any more and it'll take a very long time to cast these.
A moderate amount of health all at once to your entire group after a cast time, these are great for keeping your entire group topped off.
Don't be a spammer! One of the most annoying things, for me anyway, is when someone just sits there and spams group heal. Pressing the same button over and over while in complete safety and getting fairly good numbers in scenarios is not skilled healing. Group heals are part of your arsenal and should be combined with the rest of your skills.
Tops off, and negating AE damage. This is all these will really do. They won't keep up someone who is being assisted on, or someone who has any number of anti-healing debuffs.