I did boss fights with "sorcerer" by spamming through my casts of soul spear (and a handful of similar heavy hitters, soul greatsword etc) and then fighting with a buffed infused uchi or rapier for the other 70% of the HP bar.
Realistically though, the levels and upgrade materials are probably better spent elsewhere. I imagine you could blast off "only" 15-20% of its health bar with a ranged weapon instead.
I've come to the conclusion that sorcery et al. needn't even be in these games to begin with, and largely exist to add variety. The difficulty in Dark Souls derives mainly from timed rolls (and/or blocks and parries) and heavy-hitting enemies being in your face constantly. There has never been an enemy I defeated through sorcery for which the tactics used weren't fundamentally identical to melee tactics, save mostly fruitless attempts at maintaining a distance, and probably the fights would have ended more quickly and safely through methods other than sorcery.
I think I was spoiled by Dragon's Dogma. While not as difficult or unforgiving as the Souls series, it has its moments, and you also received two customizable autonomous party members. You could therefore play a magic user and focus on careful tactical positioning and the timing of casting spells, rather than having to dodge every attack yourself and use your spells as a slower, more limited, and shittier version of melee that can occasionally masquerade as a crossbow that uses less stamina.
I went ahead and bought DS3 since I need to de-stress from being butthurt about sorcery in DS2. Went with a knight, and effortlessly sliced through everything and the first boss without dying, including the optional crystal lizard guy. From what I've heard, doing even the beginning of DS3 with magic is far more difficult because it was nerfed even harder.
Basically From doesn't want to allow magic to do the massive damage it should do in order to justify the massive investment and paper-thin resultant defenses. I suppose once you have a lot of levels and all the good gear it's pretty okay.