Well it depends on the class. Solo play generally doesn’t have many tactics, but you still have to use certain moves at the right times like spell cancelations, stuns, and depending on your class even properly position yourself. For example rogues do higher damage from behind and mages have enough power to beat multiple enemies but they also have little health and armor so they tend to use a spell that freezes enemies on the spot for a few seconds (they can still attack if close enough) and then using area of effect spells on them. They run or teleport further away when the freeze spell breaks and the enemies give chase before repeating.
I don’t think anyone could actually be killed from the average solo play enemy even if he didn’t pay attention, especially in the beginning, but in general the challenge is ito do it in the most efficient way which will allow you to more quickly take on a lot of them as quests usually require, rather than having to take a break after every enemy to recover.
Still, party based battles is where it’s at. These involve threat levels just as much as damage and healing. Each enemy attacks the target he considers the highest threat to him. If 5 party members each attack a different enemy then they will most likely all keep that enemy on themselves until the end of the fight. However, enemies in instances or group quests are not soloable as they do more damage and have much more health so that tactic would simply not work and everyone would die.
What is needed is to have a character who will act as a tank. A tank can take a good beating and has high threat moves to manipulate the enemies threat levels and ideally have all of them attack him while the rest of the party keeps him alive with heals and slowly takes down the enemies’ hp (a tank generally doesn’t deal that much damage himself).
Being a tank what I mostly do is charge in a group of enemies (gives a little rage) if possible, and if not simply pull them using a bow. Then I use 2-3x sunder armor on all of them. Sunder armor is a high threat move that reduces the target’s armor and stacks up to 5 times.
This has to be done very fast because as soon as I have to be healed (and since everyone is hitting me it will be soon), if I have not managed to cause enough threat, the enemies will start chasing the healer as such spells cause a very high threat level to all. There are other high threat moves for the tank to use in combination with sunder armor, such as devastate, retaliate, shield bash, but I won’t bother explaining them all here.
Other members will try to reduce the number of enemies we fight at once:
Mages can turn one enemy into a harmless sheep. Warlocks have a minion that can charm enemies, or they can banish a demon themselves. Priests can shackle an undead which causes him to stay put in one area. Hunters can set a trap and then lure an enemy to them, freezing him for a while. Rogues can “sap” an enemy, stunning him for sometime.
All these - and more - come in very handy when you fight larger groups. They also all break after sometime or if the affected enemies take any damage by accident or on purpose. Banish is the exception which lasts a set amount of time and you cannot damage the enemy while it lasts.
Anyway, after doing all that, its down to basic tanking and damage dealing. The tank by this time hopefully has caused enough threat on the remaining enemies so the mage, warlock and whoever else can freely start doing some major damage while the healer keeps the tank healthy.
Other moves of course need to be used. If a caster is among these enemies then he can be stunned mid-cast or have that spell cancelled, reflected, or he can even be silenced. A warrior tank has moves for these, but other classes do as well.
Other than that, the tank must also make sure he receives as little damage as possible by temporarily disarming an enemy, stunning another and using other defensive skills which cause enemies to deal less damage (demoralising shout) or deal attacks slightly less frequently (thunder clap). That makes it easier on the healer.
Of course that’s if things go ideally. There are always enemies who do things like burn the mana of spell casters, use fear that sends all members running around while they attack whichever they prefer regardless of threat, with the tank having to quickly recover and take back the enemy. Some times you may simply have to deal with too many enemies at once, or with enemies who have immunities to spells like sheeping etc. All sorts of things that generally cause much pain and suffering and require quick regrouping and thinking and juggling of multiple enemies with different skills at once.
Bosses have their own abilities and immunities so the tactics differ for each. PvP is also a vastly different type of play which requires much different skills.
Anyway, I can’t really explain all this enough without listing the tons of different moves each class can have. These pages may help you understand how some things work in combination with the above:
Level 1-20 is basically training, getting accustomed to your character’s abilities and style of play. Things start to get more interesting at that point as I think you are able to join your first instance at around that time, though solo quests are still the majority at that time. Still, even solo I’m sure you could find at least one class that is enjoyable to play. I find paladins are more interesting when soloing than a warrior for example since they have more diverse skills.