Shields that are used offensively.
A line of units that either die shortly after being cast or are self destruct.
A heat up/cooldown mechanics, a mode switch. Like rage stacks; only when the damage maxes out the squad starts taking damage and at any time you can change it's mode and those stacks of rage turn in to a shield that scales based on how many rage stacks it had. You can use squads or spells to give those units a shield that would be stronger than the one they could normally give themselves and in turn, boost their damage (both to enemies and the self immolation) beyond what they could normally achieve.
This would give them synergy with nature, as you can heal the self immolation and shadow as you can self immolate for... shadowy reasons.
I think their weapons and theme would be steam, as in steam powered rifles, steam spraying weapons, large mechanical squads; lots of iron and metal.
Stuff like that.