Pulling Overtime - Complete 100 Portal (Multiverse) events.
These events are found in the single player menu and then in the multiverse section. There will be 3 tutorial events which count towards the achievement progress. Once you have completed the tutorials you will be able to choose from a selection of timed multiverses that will expire every so often. Each multiverse has a range of different events of varying difficulty. An event counts towards your progress only when you defeat all the fighters in the event, if you fail it doesn't count. However there is one event that is against infinite fighters and counts as completed on failure, even if you fail the first fight!Quick Solution (no rewards!)
The easiest way to get your 100 events is to go to the "Battle Simulator" Multiverse which is the planet furthest to the right. It has no expiration time so you can do this set of events forever and they all count. In the battle simulator section the furthest to the right is the infinite set of fighters. It has no completion rewards but only performance rewards for winning 5 or more fights but failing on the first fight counts as completing an event and towards your achievement!Slightly longer but more rewarding solution
Credit to VGM 007!
Choose an event and then when choosing your character press
and cycle to an AI load out. Now the AI will play for you. This is very good as even a newb can beat the hardest multiverse events this way as long your character meets the requirements. Sadly you can't 4x the speed like with AI Battle Simulator.
The first solution is much quicker so if you just want the achievement go for it however if you plan on completing all the achievements you will be better off playing the actual events with AI and getting more loot boxes, at least until you find Catwoman's Cat-Call ability.