My strategy is this:

• strong tank in front, preferrably with passive damage abilities like "thorns" (I love it!!), "retaliate" or "reflect". So it deals damange not only during tis active turn, but also when being attacked by others.
• another tank behind it.
• a melee damage with "sneak" ability, so that it can attack every turn, even when it’s not in front. But it will stay active when it’s in front too, unlike the range damage cards.
• a few magic and/or range damange cards. Those are key to victory!! Because the more total damage your cards can deliver each turn, the greater your chances for success are. Put your magic card(s) in front your "sneak" melee damage card and your range damage cards behind it.
• You can have a passive effect card as part of your team, like "weaken". But tank healer is a must!! If your tank can heal itself, it’s a bonus. But having an extra tank healer is almost always a very good thing.

Here’s an example of one such team that I’ve built. It’s been quite successful in battles and even took me to a higher league yesterday: