Core D/P isn't good. You can go condi P/D core (like SA 322, kitten 321, Trick 323) or power S/D core (something like Acro 321, Trick 312, kitten 222,. withdraw, SS, RfI, SoA, DS)
Didn't play engineer much but I like the idea of running around with a flamethrower. After the removal of retaliation and with celestial buff what do you think would make a good flamethrower build?