Honestly it's not "ruining the objective". To level up, you have to get kills with a specific type of gun.
Now in all modes I listed, killing is still a big part. Plunder even helps level Gunslinger/Triple Threat when you're the guy who just stuck with carrying the idol (unless you suck that bad with your pistol).
In TO it's fairly easy to level it up there while following objectives. It's not like you go "hey man, it's my hill/turf, please leave" and they go "oh sorry."
You go "BIIIIII- KABOOM KABOOM KABOOM PEW PEA PEW" then they go "Respawning in 6 seconds.."
Well that's Weapon Expert. I personally leveled it up across all modes, I literally leveled up that one booster to gold in every single legacy because I had so much fun doing it, so much so that it plays heavily in my main playstyle now and I even get triple threats naturally now without knowing it.
for level 1, yea I don't think you can anymore since gunslinger medals are unobtainable now in objectives modes. But from level 2-3, I believe you can still get triple threat since you don't need the gunslinger medal itself, just 3 pistol kills, 3 long gun kills, and 3 explosive kills.