When fighting any of the big bosses that cover more than one tile there are a few issues I have found.

You cannot attack move when you are not next to them.  Normal enemies if you click to attack your unit will move up to them and then attack, with these bosses I have to manually move up and then I can attack.

If you try to summon a lightbringer it allows you to cast the summon in the tiles that are covered by the boss.  When I do this the Lightbringer does not appear and my caster is stuck in his turn.  The only way to continue is to auto finish the battle.

I have defeated Torax twice and Delin once so far.  I still have not received either of the steam achievements for those kills yet.