My strategy:
Units: Dragon Rider, Phantom Armor, Black Knight, Centaur Archer, Magic Cannon, and Fire Succubus
Spells: Comet Shoot, Hercule's Strength, Phoenix Strike, and Athena Protection
In Battle:
Wait for most of you units to be summoned. Don't retreat, but call your hero if he starts to move. Shoot comets at large groups of enemies. Once Hercule's Strength is ready, cast it at your base. Then use Phoenix Strike right after. Then charge. If you hear the revenge horn, cast Athena Protection.
I would like this more if the things you need to use to envolve your guardians cost money and not trainer tokens. I think you should also be able to buy trainer tokens ith ingame money, like 200 gold per trainer token.
aim their weak points using the projectiles with curved trajectories. Or (for the shield guy) shoot right at the moment when he uncovers for a while.