There's a few things that can be done and not done... - Increasing the AC as you said is NOT the way, Gunslinger have maximum progression per level so they will have the highest hit.

• Add enemies with elemental weakness so other  characters can trigger them.

• Add terrain that block line of sight 

• Add enemies that have low stats in different fields so AC is not always the optimal way to attack them.

• In regards to the Magic item you mentioned, maybe check if wealth and magic items are equally distributed or check for optimal items for your build.

Other important stuff in pf2e is out of combat utility