When playing offline games, the AI will hide its Summoner Card in its back ranks - you should consider doing the same. While the Summoner Card is immensely powerful, the game is over once yours is defeated!
Walls can take a beating, but you should endeavor to defend them at all times, since you can only place reinforcement units in squares directly adjacent to a wall.
Using cards with multiple wounds (like champions) is a great way to tie up your opponent's weaker units. Manoeuvre weaker units of your own around to pick off those engaged with your champion.