I recently found this out myself, but if you use energy weapons to destroy the shielding above the grunt inside the mech, you can headshot it like a normal grunt. My personal go-to is the Sorrow of Teash, but there’s a few others that work good. Doing this has saved me a lot of pain.
I can’t stress this enough, but if you have damage boost and a plasma pistol you can pop the canopy by yourself. Too many players just shoot at it with either loadouts weapons or ineffective power weapons. I think the ultimate combo for mythic grunt mech especially if you don’t have enough time to take it down conventionally, is to equip damage boost and the 2nd tier laser (I think endgame). It has like 10 shots and all you need is to land 3 to drop the canopy.
There’s a few good weapons, I’ll try name the ones I usually use.
Spitfire Plasma Pistol, don’t charge it. Just pea shoot it
The Sorrow of Teash Brute Plasma Rifle. Shreds it.
Twin Jewels of Maethrillian Binary Rifle.
Endgame Spartan Laser
Selene’s Lance Spartan Laser
Probably some more I’m forgetting, but if you can pair some of these with a Damage Boost they’re amazing.
Last night I clutched a Mythic match with ZERO seconds remaining on the final round by using this strategy. Here’s the clip