How to kill the grunt boss. I suggest using the scourage of fire or any plasma caster. The next most lethal thing is anything oni. The wasp is ok.
I always try to let the enemy or other team get his health down then go in with whiplash or arclight to get the final blow. Being in a air/land vehicle works too, just keep as far a distance as you can whike still landing shots because he seems to over look 20 people in front if him to drill you in a wasp 200 yards away
Just shoot it and throw granades. 
I know its hard to kill it but maybe it will die if u do that
WZFF or WZ? It kinda differs in strategy depending on which mode.
For Firefight I use a Scorpion and snipe from afar, or if it’s earlier on in the match Ghost Ultras/ONI Hogs work well.
The one that spawns on the beach in WZFF is really annoying to kill. The grunt mech’s weapons home in way too much. They can also kill a banshee with an ultra attack in one hit.
In Warzone Firefight, I just hit it with the Plasma Caster and thrust back into cover the moment I see it shoot a projectile and repeat about a dozen times.