For his first form, charged bombs (yellow) and slashing will destroy him quickly. On his second form, Dalles puts up shields that must be taken out with the corresponding special (green, red, yellow). For some reason Toal’s fire (red) can break both the green and red shields. On top of that, it can damage him quite a bit through his shields. On his third firm, take out one of his minions and just use fire on his shield until he dies.