The best possible way to get this is to use your EXE skills after enemies have been guard broken.
Depending on what level you are and what you're fighting against, you'll want to make sure you have an appropriate amount of the EXE Gauge filled. If you're fully maxed on level, one will probably suffice. If you're going for this as early as possible, you'll want 2-3.
Once you have that completed, approach a group of enemies or a boss in a dungeon. I would recommend a boss just so you won't have to worry about multiple targets, timing, and positioning.
If you're going against a boss, guard break them (attack with
attacks until "Guard Break" flashes on the screen), and make sure the next 2-3 attacks are your characters and not the boss. Then, unleash your EXE attacks of choice. Again, how many moves/EXE attacks in a row you will need depend upon your level and what you're fighting.
If you're trying this against a random swarm of enemies, make sure there are at least three present in the fight. Wait for your party to have three consecutive attacks, and spam the EXE attacks then. Guard break is still helpful, but it is significantly harder to set up guard break and consecutive attacks with a group of enemies than with a boss. Three consecutive EXE attacks should be able to reach 100,000 by Chapter 5, possibly earlier if you over-level.
I personally got this in the final battle of Chapter 5, against
This was my strategy and party for that particular battle:
*** Spoiler - click to reveal ***
Party:
MAGES.
MarvelousAQL
Plutia (DLC Character, mostly used for healing)
My party was around the mid-40s in terms of level. MAGES. and Plutia had their best weapons and Life Rings, no other equipment changes. MarvelousAQL had all of her default equipment. Since CPU Breaker is a machine, I gave Plutia and MAGES. all magic attacks and MarvelousAQL her highest break-power attacks in each category.
Strategy:
I had MAGES. hang back behind MarvelousAQL (since MAGES. has terrible defense and MarvelousAQL is tank-y). Plutia was a little off to the side - still within striking range of CPU Breaker and within healing range of MarvelousAQL and MAGES. I hammered the boss with break attacks (aside from Plutia, who spent most turns healing everyone) until its break meter had only a sliver left and then defended with everyone until the boss struck again. My three characters had turns lined up, so I broke the guard with Plutia and used MAGES.'s and MarvelousAQL's EXE attacks to get the trophy.
Note: If you try to replicate this strategy and don't have Plutia, the closest substitute is Compa (though Compa is much more frail). If you use Compa, try to max out her HP with equipment and have her hang back by MAGES. out of CPU Breaker's range.
If you have any suggestions for the solution or comments about it, please let me know with a comment. I'll be happy to assist as best I can.