I think the OP mean when you add your Buffs and abilites before you use the weapon.
I myself as a tact captain, was in a 1V1 with somebody trying out a new setup and I almost 1 shotted him when I added by buffs, I just had to change to my MACO sniper rifle and I did exactly that, took him out in 1 shot.
I don't just think it is the weapons and buffs though, it also comes down to the type of armour and shield that you use. I use a CAPx3 shield with a Polyalloy weave armour until I finish getting my complete MACO ground set.
He's right. I've been able to do it, and I'm not even the best at it. Tacs with Fire Team or those dirty gankers with an Operative kit can do it, no problem. Combine that with a large power cell and you got a seriously big stick to hit someone with.
You want to counter them? Sci officers with the proper healing kits can take any abuse these one shotters can handle (I know... lousy healtanks need to be nerfed). Or if you're an engineer, a timely weapons malfunction/hold/orbital strike works wonders (lousy weapon malfunction needs to be nerfed...). Or if you're a tac... either try to outsniper them or go the coward's way and use your Ops kit.
Polyalloy Weave or Energy Dampening armor is a must. if you know your enemy's weapon of choice get armor and shields with that particular immunity (if the guy's sporting that retro Type III then it should be an easy call to make). MACO's great for the rechargable shield. Don't discount Omega Force set either... it does wonders for your crit severity!
And hell... you should be sporting a pulsewave and retro rifle combo yourself. There's a reason why they're popular. They're effective! Don't want to be part of the herd? Then use the classic Sniper Rifle... it's a lot slower to use its secondary fire, but it does have superior range. There's also compression pistols, which work similar to pulsewave weaponry. I've seen some really good grounders with compression pistols just waste people (and its secondary fire cooldown is faster!).
Something overlooked, however, is weapon bonuses. One of the reasons why these one shotters work so well is because they're sporting weapons with insane bonuses. I myself sport a Mk XII pulsewave rifle with a 60% at crit severity (CritHx3 I think, correct me if I'm wrong). When you've been exposed, how are you supposed to reasonably get up from that? Still, I wonder if a better crit chance or better damage would be more effective...
Mind you, I haven't revamped my grounder in awhile and I'm not the best (but I am fairly good), so take what i say with a grain of salt. Still, it should help.
I feel your anger OP...but unless they give us back the old/better personl shields, you are just gonna have to deal with it. Some tac guy new a dev, whined about not be able to kill a sci/eng guy in 2 shots or less and they nerfed the personal shields.
Thats probly pretty close to how it all happend.
But yea, after adding buffs, skills, power cells and whatever else we get, you can get some insane dmg from some of these weapons.