Some things I'd like to add:
Two tips that would help me a lot and mix things up a little if I see you in matchmaking.
1) It is pretty much a universal truth that for MWZFF, you have to be in a vehicle for Round 5 to have a chance of succeeding. If for some reason you don't want to, you can be an even bigger help by running around with [appropriate] loadout weapons and convincing the Warden of the grunt goblins to turn their backs to us vehicle users so that we can hit their weak spots. It's surprisingly easy to pull off and you'll last long than you think.
2) Since the mid-2016 balancing update to loadout weapons, the AR especially has an extremely generous headshot bonus. It takes 2-3 shots to "headshot" a grunt and 3-4 for jackals, and 7-10 for crawlers (yup, it's kinda harder for crawlers since their heads are so much smaller). Try it out in campaign if you don't believe me (they use the same sandbox), it's asinine. "headshot" hitboxes also apply to Knights'/Soldiers' sensitive areas, too and unshielded elites. If you usually have mediocre progress on Rounds 1-2 (I definitely do) because of your magnum skill, try switching to the AR. Use the attachments until you REQ up to the more familiar BR.
I actually don't even follow this tip from REQ level 2. My magnum skill sucks so I swap out for the gunfighter magnum and just spam-fire unshielded enemies. Has okay results when compared to AR burst fire. Ammo capacity is way lower (and shots are relatively weaker too compared to one magnum shot, but dps is higher), so only beneficial with Patrol Case armor mod or within proximity to REQ station. Love the fast reloads, though.
EDIT: One more.
3) There's a limited window in Round 5 with the grunt goblin where you probably don't have enough time to finish the grunt goblin off, but its health is low enough where some stars can align. So usually with about ~2 mins to go and the goblin has about 60% health, I'll whip out a sniper rifle and align myself where I can either continue taking down his console energy shield (so I can headshot him for the instakill) or the mech's nose is broken enough that the penetrating sniper bullet can find its way into the console and insta-kill the grunt (this one is way more of a crapshoot and I'm still collecting anec-data on this). Him raising his arm to shield the console interior is a good indicator that this vulnerability is currently present, but I've had mixed results. Would love to compile an Xbox DVR compilation of the times it's worked to see why this worked, though. Give it a shot and report back if you can! Salvage an almost hopeless pathway to victory! What are you saving your snipers for? WZ Turbo? lol
EDIT: two more. writing this in between WZFF games lol
4) on some maps, one of the Round 1 or Round 2 objectives is two hunters in the garage. they're usually on the top floor. if you ascend the curvy stairs, you can "headglitch" like in Halo 2 multi-player and fire your weapon at the hunters. You'll deal damage to the hunters, and since they're too close to shoot you with their weapon, they'll attempt to melee you. Unfortunately for them, they can only melee the stair railing and will continue to do so endlessly. makes for a pretty uneven fight in your favour and if you bring friends along they can finish them up quickly. It works for about 70 seconds before reinforcements figure out and flank you from the bottom of the stairs.
5) I think everybody knows that at the end of each round, remaining enemies get cleaned up by dropping dead. The kills are awarded to the person who last laid a bullet on them. Assists are similar but don't exist for enemies who are generally found unshielded. This is hugely beneficial to your REQ progress (and AI kills/medal count gambit if you're playing this). While this is common knowledge and after the round is declared "complete" you'll see your fireteam freak out and fire random shots at the remaining enemies, I often see teammates compete for the same low value enemies. If two people are competing for one remaining grunt, then only one Spartan gets the medal and REQ credit. I encourage you to see out the higher enemies with the assist medals: Elites, Soldiers, Hunters, and any vehicle to do your carnage, so that more medals can be earned through the team. Higher points and REQs for you benefits the entire team, there's no point in competing for this. I realize there's only a 5-7 second window before the end of the round to realize this but try to keep the locations of any outstanding vehicles/hunters in the back of your head as the round winds down.