The problem with Demon slayer is that most of the fights in Act 2 are not against Demons, and you do not get Instant Enemy before lvl 3. Since Act 2 is usually the most difficult part of an Unfair playthrough the value of Demon slayer declines unless you hire a merc just before Drezen.
Aside from the bugs, what parts of Act 2 do you consider difficult? Honestly curious, I've never had problems with any of the Act 2 boss fights on unfair. Gargoyle boss is easy, Vescavor queen is easy. The final fight with Staunton is only like 35 AC. Only difficult fight that you can't put off until later is the Demon summoned in the Drezen basement which gives like 15k XP
IMO Shield Maze is by far the hardest, especially Hosila (the only time I beat her legit TB is with a random x3 x3 charge crit that I posted a few pages back, every other time I've had to abuse RT), with a few Act 1 fights being tough too. This is also the point where a number of your party are chronic underperformers, like Camellia and Daeran doing fuckall for damage before you get mythic finesse, or your nuker getting fireball + bolster + ascendent element fire to actually do more than tickle things.
Also, sure, not many favored enemy bonuses early game (though I'd argue they cover most of the difficult Act 1 and 2 fights). But pets are also at their most powerful early game right after you've gotten your full party together to start buffing them. Boar can reach like 45 AC by the start of Act 2.