You don't really need a tank in this game.
A tank is a guy who'll charge in, get everyone to focus him and still smack a few people about while doing so... a tank is just another form of crowd control.
The most important thing in D:OS is definitely control, similar to chess you can have equal resources in a fight, but it's up to you how you use them to get control.
Knights - Get Man-at-arms to 5 and weather the storm, this will reduce incoming elemental damage considerably. If your knight can charge across a minefield and still be over half health, you're doing great. (especially if the charge successfully knocks em down!)
Marksmen - ranged silence/charm, super arrows for the right situation (setting up a nice stun cloud over water is always fun, as is simply shooting the oil barrel with a fire arrow).
Scoundrel - charm/stun/knockdown/blind/bleed/weaken/infect/decapitate, all in a days work for rogues. Cheap attacks, cheap CC (often you can get 2 off in a single round) plus a deployment spell. And remember, scoundrels can be marksmen in the same build!
Mages - Mages offer a lot of versatility and it's all about picking a couple of schools to combine to make your mage awesome.
Pyro - DPS, more dps and pretty much nothing but dps right? Well, only if you ignore the fact fire mages can haste anyone (wildfire on a frozen guy = no more frozen too remember) plus a nice summon to block the enemy archers.
Aero - Teleport, obviously. Great stunning capability to CC the board. Also gets a way to clean the board up if needed. And a summon
Hydro - healing, which everyone loves, plus CC and dps with ice shards. On top of this... RAIN! because rain makes stunning and freezing much easier. And a nice summon with CC.
Geo - Poison, defense and a few summons.
Witchcraft - I've yet to test this out, if someone wants to let me know the benefit of this.
Put your party together with CC in mind first, damage second, and you'll go far.
