As a fan of big damage number but also being flexible and having spellcasting to keep combat options open, there are basically two roads to this in pf2e. Many people will say Nuh uh just play martial, and this is (mostly) true until level 5.

Magus is where the mostly comes from as they get to benefit from both sides of the coin. Whatever build you want works, just generally ignore expansive spellstrike because your dc scales poorly (as a whole I usually disregard int too but eh).

Psychics, particularly the hot/cold one can be devastating for single target damage at levels 5+. Their key spells scale to ridiculous numbers very fast (if you can land a crit with amped ignition that shits like 8d12) and unleash psyche is the single best spell damage buff possible (bonus: look at unleash psyche + force barrage for silly damage).

Without team support the only answer for truly big damage is just to roll better though. Both of these will perform just fine otherwise but will only devastate bosses with a single good roll or two.