Just play adepts like you play them on gold and span warp-throw biotic explosions. you will never need to upgrade a gun ever.
As for infiltrators.....ignoring the fact the BW is horrible in comparison to the multiplayer variant, it will still tear through anything in the game.
.....
Now that I think about it, i think the only class that is difficult is the vanguard. the game's insanity in comparison to Mass Effect 2's is a [censored] joke. Don't even get me started on when i jumped into an Atlus for the 1st time and I felt like god of the ants.
Edit: Also - seriously how are people having difficulty getting war assets?
My EMS - 5875 - 1575 (multiplayer promotion bonus)= still more than what people are complaining that all they can get.
Even taking away the WHOLE 835 EMS bonus I receive from curing the Genophage, I'm still sitting on 3,447.