Just did this with Scottish Nutter and the game took less than an hour - all the other solutions seem to take longer, so here's what we did!
The player going for the achievement (Hereafter referred to as Player 1) selects Prophet of Regret, their boosting partner (Player 2) picks Sgt Forge. As soon as the game is loaded Player 1 builds a Temple (to make the Prophet spawn) and surrounds the rest of the base with Warehouses, upgrading to Blessed Warehouses when resources permit, and also upgrades the base and adds extra Blessed Warehouses in the new slots.
Meanwhile, Player 2 is surrounding their base with Supply Pads (as you're playing Sgt Forge, these will automatically be heavy supply pads) - again, upgrade the base and add the extra Supply Pads as soon as resources allow.
Both sides will need plenty of resources to make this solution work, but Player 2 will need more. Rather than waste Resources on building offensive units, though, Player 1 should take the Prophet and clear the 2 rebel bases (using leader power if necessary, Y button for those who don't know) near the middle of the map, letting P2 know when this is done so that P2 can send their Warthog over and claim the Base Locations. P1 should return the Prophet to their base to prevent him shooting P2's 'Hog.
P2 should now create 2 more bases identical to the first, fully upgraded. P1 might want to upgrade the Prophet's Cleansing beam, then once the bases are complete go get a coffee each as you'll need to allow 5-10 mins for resources to build up. Stage 1 is now complete.
Now turn on any debuff skulls you have unlocked. P1 should pick one of P2's Supply Pads and move the Prophet a short distance away. UNLEASH HELL (well, a Cleansing beam anyway!) This should make short work of the Supply Pad, and as soon as it's destroyed P2 should build a Barracks (cheapest building available).
As long as P1 leaves the Prophet's beam running, this will be destroyed almost immediately, before it's even complete. P2 just needs to keep attempting to build Barracks in this spot as the Cleansing Beam fires on it, earning P1 10 points every time a building is destroyed (it's 10 points no matter which building is used, and irrespective of whether construction is complete or not). Doing this will rack up points very quickly. though if either player begins to run short of resources, just stop for about 5 mins and they'll soon build back up.
Given P1 will finish with a Combat Bonus of a 7x multiplier, a score of just under 3k should be sufficient for the achievement to unlock. However, to be on the safe side, I'd continue with this stage until P1's score is around 3,300, then have the Prophet completely wipe out all 3 bases for a further 240 points. If you need to check P1's score during the game, this can be done by pressing the Back button.