Single player here, building up a hybrid sorc to be my first main farmer for my holy grail.
Maxed skills:
Hydra
Fire Mastery
Frozen Orb
Ice Bolt
My question is: should I invest in Cold Mastery or Fire Ball? I don’t have Infinity yet and probably won’t have one for a long time.
Get cold mastery to -100. Both Orb and Hydra have very low synergies, which is a good thing for dual element sorcs, we are not missing out much, by not having synergy points.
What you want to do after is get +skills, like skiller charms (And +skill gear of course, as high +skills as you can). They add both to the spell, and to mastery, and so increases damage much more than put a point in synergy. If you can get -fire res on stuff then that is also strong (Mang Song, Flickering Flame, Unique Jewels).
There’s a guide at Icy-vein.