I usually do tank in the middle, healer/support in the flanks, high damage/snipe heroes on the wings. Tank characters are the ones that will usually have high HP and/or defense, with skills that heal, give defensive buffs to them and/or others, and possibly quick to generate mana, since they are hit most often with tiles. Flanks usually get hit the next most often, so having heals and/or buffs/debuffs is good. Then having some damage to end your opponents in the wings.
My main team is: Tank: Queen of Hearts, Flanks: Vivica Costume and Lady of the Lake, Wings: Alice and Freya. Queen of Hearts has high HP and Defense, her special summons a minion and gives her defense against special skills +76%, while she has a minion up she has Taunt which forces all special skills to be directed only to her. Vivica Costume heals, removes buffs from enemies and gives them reduced defense. Lady of the Lake heals and summons a minion on her and the two heroes next to her, which sap mana from an enemy when they are hit by her minion. Alice is my Snipe, high damage and applies reduced attack on enemy hit. Frey gives full alliance defense buff and summons a minion on everyone in my group. Her minions buff the attack of other minions on same character. Most of my damage ends up coming from a barrage of minion attacks each turn. In raids, if I can defeat an enemy or two, I can “ghost” tiles in the gaps for higher mana gain for me while not having to rely on tile damage as much and avoiding feeding the enemy team mana.
There are so many cool and amazing team builds you can make in this game. Look at what you have and come up with skills that work well together. Avoid duplicate buffs and debuffs as they don’t stack, the most recent just overwrites the other.
Defense buff, elemental defense buff, special skill defense buff, and Paladin class Protect defense buff are all considered different buffs. So in my team build, it is possible for Queen of Hearts to have Defense buff from Freya, Paladin Protect, and Special Skill Defense all at the same time. It is awesome to see someone use a snipe on her and barely do any damage to her.