I would suggest using 2 tanks with 30% physical resistance and stone skin always on that will hack at the boss. Paladins and Knights are a good combo. You also need 3 good units to destroy the towers. Royal griffins are a good idea due to their high magic resistance. However, don't have them summon angelic guards, as this provokes the boss to use his mass attack much more often. The other 2 units must have 150+ HP and decent physical damage (towers have 0% physical resistance).
The next thing you need is to stock up on items that grant physical and magic resistance. If you don't have such, it may be a good idea to do some more fighting and open up all maps before engaging Gremlion. The Inquisitor's Sword will also help, because you'll be able to bring in more paladins to this fight. The dragons are not a good choice because they can't be resurrected properly and as soon as you lose a few of them, you'll grow weaker and weaker. Shamans may still do well, but you will need to use them for their melee attack, because if you summon a totem, this will again provoke the boss to use his mass attack, and the dancing axes will not damage the boss or the towers substantially.
In a nutshell, try using 2 tanks (Knights + Paladins, for example) near his left and right hand, and have stone skin on them at all times. The other 3 units of your choosing will have the sole task to destroy any tower he spawns. Equip all + physical and + magic resistance items, especially the magic resistance ones. If you don't have such, try to find some. If there aren't any, gain some more levels and tackle the boss on lvl 50-52.
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