Fire Warlock Easy Peasy thru normnal but

what happens when you reach fire immunes?

Ring of Fire + Fire Wave are brutal even against Fire Resistant. But Fire Immunes, whioch are thick in Hell level...what is he suppose to do?

Same as any other elemental build:
Rely on having a sunder or bring in some other form of damage.

For a lock, you get a fair bit of easy phys dmg added via a proper bound demon + merc but since both of those are largely single target, I wouldn't want to rely on just that for fire immunes, considering how plentyful those are.

A high level

Apocalypse paired with more minus enemy fire res will break at least some fire immunities. For the others you need a second damage type. Scaling magic damage will not be strong enough. Maybe go for

Echoing Strike or put some points into the Demon/Minion tree (one bound demon,

Goatman, one consumed monster). The

Corpse Explosion from

Sigil Death could also help out a lot.

You could also use

Death Mark to warp your demon onto enemies and

Blood Boil them. Idk how viable that will be in hell, I just have one point in each for now.

I'm in the same boat here. I almost finished Normal with no points allocation (only a few in Strenght to equip some armor). I have almost 200 free and he slams everything with almost no gear.

But from Nightmare and especially Hell we have to find a way to go through.
I play from scratch this time I don't have any Sunder (I mean I have but I don't want to give him) and for Infinity... way, way long road until there - I play offline SSF.

So let's see what we can do with spells.