Defiance Free to Play

Gonna add locations for the enemies. Obviously as most everyone knows you need at least 4 or 5 high level players in the area to take it to threat level 10 and spawn the apex/formidable/ark variants.

Edit: Tweaked slightly with a couple additional tips from Jackanape.

Scrapper Omnivolts - just run Scrapworks Salvage on Expert. You'll get tons of these. You can also do scrapper major arkfalls, but you have to get to the bonus waves on the small crystals and be at threat level 10, so running the co op map is easiest

Volge Viscera - Either the incursion/siege that ends at the quarry (the one southish of Ken's farm, I'll nail down more specific location later) or the siege in San Fran that ends at the Bathhouse. Also the Major Ark Impacts that you have to place 4 batteries at, run the outside area after the initial waves of enemies, when the volge show up.

Mutant Minigunners - Whenever you see a Major Ark Impact go place the 4 batteries and see if mutants come. If they do, recruit some high level friends/clan members to come down and bump it up to threat level 10. You'll get lots of the big blue bastards during the outside area before the volge show up. Also the Incursion at Mt Tam that ends at the academy is good for these (if it shows up). Lastly there are 5 in Liberate the Lost on Expert.

Raider Tanker - If you can get a minor arkfall where it says to destroy the crystal with raiders, just get it to threat level 10 and keep farming the raiders rather than destroying the crystal and you'll get lots of Tankers. Also like the Mutant Minigunners, the Major Ark Impacts may have these (the outside area is either Mutants, Raiders, or Dark Matter before the Volge show up).

Hellbug Monarch - Do the Sausalito incursion and at the end/siege phase you'll get 4 Monarchs every other wave. Most of the high level experienced players tend to congregate around the gate area, so try to pull the Monarchs up to that area and they'll go down fast.

Infected Hulker - the Formidable Hulkers at the Kenn's Farm seige, or the incursion that ends at Kenn's Farm, will count as long as it is at threat level 10. The key here is to make sure everyone is attacking the hulkers and ignoring all the smaller enemies. Two hulkers will spawn on waves 4, 7, 10, 13, 16, 19, 22, etc. They tend to spawn around the half-way point of these waves unless people are killing too many small enemies, in which case they may spawn at the very end leaving no chance to kill them. Your best bet is to go to this siege and congregate on the road that leads away from Kenn's Farm (the road that runs between the small house and the big barn). I'll give more specific info later, but most of the experienced players already know where to congregate on this road, so just look for the giant gaggle of players and stay there. Make sure to target whichever hulker has the least health so that you're working with the majority of the players. A good group can kill both hulkers before the wave ends.

Darkmatter Bullwark - The Commander Cronkite co op map spawns 4 of these at the boss fight on expert, and you can lure them back up the tunnel to fight them. Alternatively if you go to Darkmatter Major Arkfalls you can get them during the small crystal portions if it gets to the bonus waves and there are enough players in the area killing everything fast at threat level 10. Finally, like the Mutant Minigunners and Raider Tankers, the Major Ark Impacts may have these (the outside area is either Mutants, Raiders, or Dark Matter before the Volge show up).

99er Goldrusher - Motherlode co-op map on Expert. Jackanape said he got 14/15 in one run.