Ladies and gent, I've found a map to farm Grineer Manic (credit to Jhurst89).
Now here's my tip to farm him:
Before going to farm him for the mod Four Rider, equip maxed Sure Footed and Fortitute (resist knockdown, he will knock you down a lot, by equipping these 2 maxed mods, which can be easily maxed, you have 80% chance to resist knockdown).
Now, once you're set, go to Phobos -> choose the Exterminate mission Clustril, play and keep killing enemies, now, the most important part for him to appear, LET THE ENEMIES TURN ON THE ALARM
, or else he won't appear, you can attempt to unlock doors, but don't hack alarms, I repeat, do not hack alarm, then just go onto your business and killing enemies, guaranteed 99% he will appear a few minutes later (tested
5 times), unless you kill and extract too quickly.
You may try and bring a Nekros and try to Desecrate him for another chance to get the mod (
not tested yet
). Tested, Nekros can Desecrate him.
P/s: if you've killed all the enemies and he's not appeared yet, check if your alarm is on (highly suggest you play solo or invite friends that know how to do this, others might just turn off alarms), or wait for him.