If you get chairman hes a huge help, cycle speed, chair, speed, chair, speed, kami, speed and you will get a really fast result (about 40-50 seconds) also this is with max lvl 14 base
I used 4 units and it’s not that fast but still works. Units: Speed Battler, Kamikaze Battler, Assassin Battler, Chairman Kamikaze and Chairman are level 20. Speed and Assassin are level 15. Bank used: Blood Altar. Base used: Slammer, but any base works tbh. Time taken: 58.61 seconds (Might be lower as I ended the timer slightly later)
3 star doombringer too slow lol my fastest run of 3 star doombringer was 6 minutes, the average xp you get for each completion is roughly 15k so divide that by 6 and you get 2.5k xp per minute which is only half the xp of the rocket arena farm