That's when you use a merc equipped with Infinity

Yeah but that isn't cheap. The whole point was a fishymancer can do the content without exorbitant cost. Also, the conviction aura from infinity can't even break a lot of fire immunities in hell difficulty so it doesn't fix the problem unless you also have a pocket necro casting lower resist curse or you have some other means of casting it. Even if you manage to break the immunities, you have to account for any remaining resistances. You can only break very weak immunities with infinity alone. Any monster with 120 or higher fire resistance isn't going to have their immunity broken by the level 12 conviction on infinity. The only monsters with immunities that you can break are monsters with 115 fire resistance, and even that will only get dropped to 98%. So have fun with that I guess! You'll still end up with around a third of the enemies in hell difficulty being immune to fire.

http://diablo.gamepedia.com/Immune_M...blo_II)#Fire_3

EDIT: Corrections because I was looking at the wrong info originally.

For anyone who still doesn't believe a fishymancer is superior to a sorc (of any flavor), have a look at this.

https://diablo2.diablowiki.net/Guide...ar_-_Untwinked

I've followed this guide, and I've played fishymancer completely self-found. A fishymancer in mediocre gear can solo any of the standard content in the game (not true of Sorc by a long shot) and with good gear can even solo ubers (I don't think any variation of Sorc in any level of gear can solo ubers.)

EDIT: Looks like a pretty obscure and atrociously expensive melee sorc build can do ubers. https://www.diabloii.net/forums/thre...istram.536884/