I'd once planned on writing a tactics guide to some of the more popular scenerios, thinking that perhaps telling people that the bauble holder needs a healer, and to stop bumrushing order, or that the part runner is very likely to get cut off half way to the turn in and needs more than just my sorry DoK butt to guard him it might do some good. But Regis made me realize that I think we need to go even farther back to the basics. If people would just grasp the basics then perhaps we can move on to the "complicated" stuff.
So here's my list, in addition to Regis' and with some things likely restated because I think they need to be stressed.
Open your eyes and look before leaping
Take two seconds after you encounter the enemy to scan the crowd before you rush in. Note first the make up of the group, the classes there. This is where is pays to be able to pick out the classes of your oponents by sight. Ugly hats = witch hunters. Giant hammers = Warrior Priest or Ironbreaker. I won't get into describing the visual difference between dwarves and humans because I just had lunch. (Haha! Dwarf insult for this post finished). If the group is melee heavy you can be certain that they're going to rush your lines very soon. Take note of where the healers are, how well defended they are and if they're grouped together. Healers who stick together are hard to kill, we tend to look out for each other. That straggler goblin attempting to hide behind a rock over there is easy pickings though. Don't simply target the first person you see and never run blindly toward your enemy just because you saw red on your screen. You're not a bull.
Stop. hitting. the tanks.
A tanks job is to ... tank things. This is the guy that can sip tea and alt tab out while taking hits from that hero mob that two shots you. His job is to absorb damage and if there are any other classes at all on the field you should not let him do his job. Tanks are the last things to be killed in most cases, focus fire is the only way to take them down and if you either a) attempt to solo the tank or b) attempt to focus fire the tank while there's plenty of other targets one of those other targets is going to rip the crap out of you and/or your teammates while you're doing a two step with their Black Gaurd. I know it's really hard to see a dwarf charging at you and not desperately want to smash him (Two!) but if you do then he's doing his job. He's distracting you while you could be doing something far more deadly like killing his healer or the witch hunter who's busy killing your healers. Ignore the tanks until there's nothing else to hit, it annoys the crap out of them. And for cripes sake never try to melee a tank with a shield face to face, attempt to get at their back with everything you've got. Parry and block do not work from behind so coming from behind leaves you only with their several thousand armor to deal with.
Use the terrain
Scenerios are really well designed battle fields. All those trees and rocks, those little hills and sharp turns aren't just there for looks. This is very individual to each scenerio but the basic hint here is to pay attention to everything that's around you, not just players. In Tor Anroc for example, there's a small lip you have to pop up over to gain access to the central bauble crater. It's possible for a sorceress to stand just out of line of sight under that lip and drop Pit of Shades from realtive safety right smack into the middle of the battle. Also in Tor Anroc there are several paths to take to the crater and to the enemy camp. Two or three witch elves stealthing up one of these paths from behind can almost always focus fire a bauble holder down before he knows what hit him. In Serpant's Passage if you're being chased by the enemy go directly into the cart holding the turn in, this keeps you safe from any attempts to knock you out of bounds and nicely bottle necks the enemy trying to kill you so your team mates can better kill them.
In Morkain Temple there's several little places along the broken walls that offer a fantastic pocked for ranged DPS and healers to stand and still have line of sight that gives you an extra minute or two to do your thing before the enemy notices you. Some of the most fun I've had in scenerios involves hiding in one of the full coverage pine trees in Phoenix Gate and nuking or healing without a soul noticing me. (Warning: When the tree shoots a doombolt it's a dead give away. Stick to instant casts.)
There's tons and tons more I haven't mentioned, and even more I'm sure I've never found. The basic hint here is: Never go exactly where the enemy expects you to. Take another path, stand in another spot.
Last but not least, a friendly word from your healer:
I know leaping around makes you feel really cool but I can't heal you when you're pretending to be a rabbit.
Lots of healers, self included, do a fair bit of healing via direct targeting. I can see the health bars over the heads of everyone in front of me and find it often easier to pick my targets that way. I see the witch elf over there getting the crap beat out of her and I'd love to keep her on her feet for a while longer so she can poke things but I'll be darned if she isn't leaping around like a jack rabbit. I don't like playing the click ten times and miss while trying to target someone to heal game and if I miss you due to your inability to keep your feet on the ground you're not going to get the heal before you die. This also goes for running directly up to me when you need healing and jumping in my face in some attempt to get my attention. I see you, honestly I do, and I'd heal you if you weren't leaping away from my curser every time I try to target you. Feet on ground please.