Really fun game, definitely had me hooked for the few hours I was playing it. For most of the challenges, it was fairly straightforward to complete, having a decent variety of options to choose from. Most of the challenges I could get 4 stars on using a bolt action or high powered sniper. The exceptions being defender and time attack.
For defender I used the Stoeger with maximized penetration and buckshot, with the maxed scope and grip, honestly, this is just a strong combo as the smart spam clicking will clear the screen for the most part. Time attack was a whole different beast which required some experimentation and planning, The snipers and LMG's were just too slow to bring out the points, with SMG's being pitifully weak, initially, my shotgun combo seemed to be promising if I could edge it out with a launcher at the last moment, but this proved too difficult for me. Ultimately I used the FG42 with Greed, ROME05, FMJ, Grip, and Accelerator, paired with the MK 153 SMAW with High explosive.
I feel like all of the weapon classes are different enough to give the game good variety and the challenges also help with this. The armored enemies and penetration mechanics do tilt the game towards the higher powered weapons leaving the machine pistols and SMG's wanting a bit.
All in all this game is very well polished and feels great. Good variety and most weapons are quite useful in the challenges with non-of them being outright un-usable for me.