While ive been on my engi finding tactics on how to defeat the other classes and i found a good but somewhat cheap way,but is a highly chance to kill, first is tactic for op
1.get behind wall amd use my protection, i try to be quick so the player doesnt get agervated and run around pillar(never use suppresion yet it,usually the op/engi will stop you) i wait for the op to miss there stun.
2.you charge and use our 3 dots and pain and run back to wall and heal before the stun recharges. Heal and wait for the damage settle in.
3. Last the op is weak and useless charge again and use your build(if they use there buff and uses amplify pain use your heal to repair it so they do less critt and damage.)
1. For engis what you do is i wait for them to use one or two of there dot skills and i charge at them and keep them close and dont Let go from there you should have killed them, i killed 3 plat engis with a red engi set to kill them with 30 hp

Ive hoped this can help all the engis out there, as soon as i find a tactic to killing a comm i will post a another. Please comment if this works out for you(this will take practice and its really good when used)