You can kill just about any Grunt Goblin within a minute or two, even on Mythic Firefight, just by using a Plasma/Energy-based weapon to take down the shielded canopy, then nailing the headshot. My personal favorite is the Endgame, which can be used to take out the shield of any Grunt Goblin with enough accuracy. Just make sure that most of each shot hits the shield, utilizing cover and patience. It obviously won't go down in one hit, but the Endgame has a ton of shots, so it's pretty forgiving. The Spartan Laser is a good alternative, and all Brute Plasma Rifles will work as well. Then, when the shield goes down, try to get in a flanking position so you can hit the driver in the head. I believe that is the only way to kill him when the shield goes down. It's very much a skill tactic for taking down the Grunt Goblin, which makes fighting them really fun in this game. Multi-stage bosses with alternative methods of killing them are very, very entertaining to fight. It's also part of the reason Warden Eternal isn't so fun. The only way to kill him is just to spam high DPS weapons and vehicles until he dies. The same can be used to kill the Grunt Goblin, but it's much more interesting to try the headshot tactic and pull it off successfully.
I use this method all the time in Firefight. I typically land the killing blow on around 85% of the Grunt Goblins I encounter just by using this method. Feel free to try it out and see what you think. BTW, any Plasma-based weapon will work, which means that you can sometimes scavenge Plasma Pistols and Storm Rifles if you have no Plasma-based REQs.