On some levels, the enemy sends firewarriors in balloons that have the shielding spell cast on them. They seem to become immune to firewarrior attacks, lightning bolt, fireball, insect plague spells and since they are in a balloon I can't even use the swamp spell.

Although not a major problem, but a minor annoyance, I'd be grateful if anyone has any tips on how to get rid of them.

I found a way if anyone is interested. Cast Swarm, however don't cast directly at them, since their shield will deflect it; instead cast it just next to them.

Hypnotize also works.