FINAL FANTASY VIII

When?
Best done in the endgame, after acquiring the Ragnarok in CD 3. If you are also going for the Low-Level Squall achievement, create an extra-save on CD 3 before doing the last story steps to return later, after you finished the game.
It can be done in CD 4, via escaping the castle in the second screen with the middle portal to the left. It leads you to the southern Centra continent. Now find the Chocobo forest in the area, take it and ride it via the shores to the Kashkabald Desert (south east, very close to the Cactuar Island). You will find the Ragnarok there.

Preparation
You should use the following team: Squall, Selphie and Irvine. Squall has 255% hit rate by default, Selphie also with her ultimate weapon. Irvine because he has one of the faster attack animations, but he needs a Hit-J from a G.F., meaning Cerberus, Diablos or Eden. If you have a Hit-J-Scroll, you can use any G.F. of course. The spell Triple is recommended for Hit-Junctioning. You could also go for Initiative equipped on the team, but the F2 booster takes care of that. Don't forget to take out Elemental-Attack Junctions.
Once your team is set, if you are willing to use the boosters (trust me, you want to), this is what you need to do: Activate the Speed Boster (F1; faster passage of time), Assist Booster (F2; keeps your characters alive unless they get a hit 1-Hit-KO) and 9999 Booster (F3; guarantees full damage, making your strength irrelevant). Now park your Ragnarok on the Cactuar Island, as close as possible at a tip of the island. This island has the quickest spawn rate of monsters in the game accessible at this point.

Active version
Run around, try to stay in the desert, not the rocky regions (higher chance of more cactuars to spawn in the battle). Hold down X and clear out those cactuars.

Idle version
This needs the magic of AutoHotKey, a famous program to create simple or very advanced scripts to automatically. A script could look like this:

F8::
tooltip % "started"
Pause
send, {Left DOWN}
loop{
  send, {x down}
  sleep 5
  send, {x up}
  sleep 5
}
F9::pause
F10::
send {x up}
send {Left up}
F11::ExitApp

Basically, the script holds down left for you at all times, and mashes X a lot. That is all you need. Don't use right, because with the booster activated, you will do Limit Breaks, which take a lot of time.
Stay as far away as possible from the Ragnarok. Equip Enc-None and just hold left for a while to check that the circle you will run never gets close to the Ragnarok. If you are happy, activate the script with F8, then press F9 to see the magic. You must not change the window or get out of the game. To stop the magic, press F10.