Just take a look at the tavern, assess the stats, and try to recruit Marbas.

As I understand it, the opponent has Deleb. Marbas will be significantly slower in terms of tempo, to put it mildly. Even if it's him, it's one thing to defend against single chaos demon spells, and another to defend against three spells from an elven barrage (plus, in the first two or three rounds, the elf's dryads can help).

What's the chance of getting spellcasting for demons?

They have 8, the elf has 4. But the elf has 8 for leadership (the demon only has 2, which isn't serious), and empathy is no worse than spellcasting.

In my opinion, it will work if you level up the second warlock.

Warlocks are unmatched in chaos, but anyone can level them up if they get lucky.