I found him very useful, except mostly as a complement, not the main killer, since 99% of my malfunction runs have had a dominant Marksman whether it was me playing him, or somebody else. The build to use is: freeze resist, bait armor, heavy capacity, heavy charger, reflect shredder.
Now, he isn’t very useful until he gets a trishot or salvo, but the upside is he (shouldn’t) need any blue box ammo, and he doesn’t need to fear the freezing deebees like everyone else does. Maybe he carries a gnasher and enforcer just to ward off trackers and rejects at first. In second big encounter he should get a trishot or salvo. Now, he can carry the team to the saferoom if the Marksman happens to have a hard time or die (I had this happen once). Since he doesn’t rely on any guns, he is perfect to retrieve a longshot or gnasher should the marksman or infiltrator die.
Second act he should get another heavy early to tote with him. Then at stump, he can run out with his ult, make short work of the stump and hopefully most of the deebees around it too. His ability for making that stump fight as short as possible is very underrated. Now he should have a trishot and a salvo with 30+ shots to help with the final big fight, but again, Marksman still carries it here. He at least needs to kill one of the sents. If he flubs up and fails to kill one of them, then Gunner may still be able to take one out with his salvo or trishot though.
Well, then you’re a better striker/blademaster than probably 99% of players. I don’t think I’ve ever gotten one of those on my team who did anything notable in this hive. The three classes I’ve done Malfunction with since it became the weekly are Marksman, Gunner, and Demo. Only with Marksman did I do the brunt of the killing though and I think either he or Tactician are mandatory to guarantee being able to deal with the sents.