for a firesorc,holy freeze merc is the way to go,u can aim ur fireballs better when ur enemies r slowed no? =) , nah inferno is crappy as ur main killing skill would be fireball or the actual meteor dmg not the fire dmg after it,so its juz a waste of points as kuja said u can allocate those skill points into hydra , no use putting on another elemental tree as it would do little to no dmg to the monsters in hell