What's the 'proper' way to tackle the combat?
severe case of needing to git gud
https://rpgcodex.net/forums/index.php?threads/mask-of-the-betrayer-questions.110158/#post-4668069
Warlock is absolutely fine for MotB, in fact, since you don't have to rest and can go on and on it might even be advantegeous in this campaign.
If you have trouble you're probably not using Safiya very well. Give her the robe that can cast shadow shield and use Shadow shield, use improved invisibilty, persistent haste, a spell mantle to defend against those wizards/ some sort of breach to breach their defenses. Stick her spell slots full with disintegrate/empowered disintegrate because as a transmuter RWoT she will get +5 DC on transmutation spells. Buy the +6 int ring for her. While leveling up pick scorching ray and firebrand as spells so you have good lower level DPS spells on spell levels where she hardly knows many spells yet.
Get bigby's forceful hand spell. Typical i win button.
As a warlock you really only need the following feats: (optional 1st level feat spellcasting prodigy), blind fight (mobs with concealment are annoying) , shield proficiency to be able to wear a zero arcane spell failure shield (Safiya has all the crafting feats, craft a mithral heavy shield or zalantar light shield or use one of those 0 ASF shields available in MotB), combat expertise and improved combat expertise (requires at least 13 INT) which is basically free +6 AC 24/7 because the attack penalty doesn't apply to ranged touch attacks. You might want to take Able Learner early to easier increase cross class skills.
Get a +8 CHA item for your warlock.
invocations:
1)
Dark One's Own Luck (gain CHA mod on saves)
Leaps and bounds (free +4 DEX and tumble, 16 native DEX+ this let's you max out mithral breastplate which is your armor of choice)
Beguiling influence (free social skills, useful for MotB)
2)
Eldritch Chain (make your blast AoE)
Brimstone Blast (add some fire dmg)
whatever you want, normally I'd say flee the scene but in motB you can have Safiyas peristent haste, so it isn't that important
3)
Vitriolic Blast, not subject to spell resistance, so your goto blast when fighting mobs with SR
Devour magic
Bewitching blast, see how all the enemies are confused all the time
4)
Binding blast, upgrade to bewitching (which you could now switch out for something else if you want) , see how all the mobs are stunned 24/7
Dark Foresight
Retributive Invisibiity
max Spellcraft so you can take Eldritch Master at lvl 21 (blast dmg x1.5). Craft a mithral breastplate, stack regeneration (fire essences let you add regeneration on armor and shields).
Improve AC and saving throws (see link above).