Well, you need units with decent magic resistance and good damage/HP ratio.
Since his mass attack is magic based, an item like the antimage diplom (which is great for a mage with its +2 intellect) from the university in Elon (near the pier and the mage tower, where you land) will work wonders for you. His hand attacks are physical based, so 2 units that have decent physical resistance will do well as main attackers, while the 3 additional units will destroy the towers he spawns.
I propose this lineup:
Paladins
Knights
Horsemen
Guardsmen
Archmagi
Why?
Having non-retaliation units to hack at the boss will actually encourage gremlion to use his mass attack much more often. Having 2 high phys resistant fighter units (like paladins and knights - both have 30% phys resistance base) with stone skin near both his hands (1 each) will minimize the losses you take with those 2 stacks, while the rest of the stacks mop up any towers he summons.
In the battle, use calm rage spell when no units need stone skin or other buffs to gain mana, which should be used for phantoming the paladin stack to ress the losses you suffer with your weaker units.
When he's low on HP, stall. Wait for him to spawn a friendly gremlin tower. Now kill him. Group your units near the friendly gremlin tower and go "phantom the paladins" crazy. Use the phantomed paladins to ress your troops till your out of mana and rage. When you're out of both, kill the friendly gremlin tower and see how you did. If everything went as planned, you should be able to do this battle without losses.
If this does not happen, you may as well wait till you're level 50+ and try your hand at him again.
__________________
|