Notes from source deck:

Upgrades

Priority 1:

1x Adaptable As you tech quit hard for scenario 1&2, Adaptable allows you to adjust your deck later.

2x Flamethrower This is the the card that will make fighting monsters trivial. Lategame enemies are usually 4/4/x stats, and on expert you need no avarage a +4 to pass a skilltest. You should calculate the odds for very important skilltests with the symbol tokens and the content of the bag, but for deckbuilding purpose, having 8 fight vs 4 fight enemies and 4 damage against 4 or lower hp enemies is amazing

1x Charisma Once you end up with 5 allys, it will be very good.

1x Stick to the Plan Make sure you have Emergency Cache, Extra Ammunition and Prepared for the Worst under it.

1x Extra Ammunition Flamethrower is so strong, that you want as many shots as you can.

Priority 2:

All solid cards, but are not really important.

2x Beat Cop

1x Key of Ys

Stand Together

https://arkhamdb.com/decklist/view/9515/the-man-with-the-golden-flamethrower-expert-duo-tptc-1.0