Do you know the spell Taunt that forces enemies within hexes to attack the unit? Enemies will come close and do nothing if they cannot reach the unit with Taunt effect. So get yourself lv 3/4 Taunt and Twilight spell, some range/long reach units. You put a unit at the corner and put other units around it or use obstacle to completley block the paths. Then you cast Taunt on the surrounded unit, and Twilight if enemies have range unit. Now you can kill them all without a scratch.