I found the best way to do this was, like the "Hail to the King" trophy, use Remote Play and a Auto Hot Key script and just leave the game running. This takes a long time to unlock but once everything's set up it's completely automated so you can just let it do its thing for 24~ hours. If you're starting from a new game, this will require a bit of prep work.
First of all, you want to get to the part where you play as Steiner and Marcus as you're escaping the Alexandrian dungeon (in what would have been Disc 2 on the original PS1 release). Along the way, collect all the Stellazzio coins and hand them over to the bird lady in Treno for the Bloodsword. After swinging left and right repeatedly and breaking out, navigate to the centre-left of the screen; you'll notice Alexandrian soldiers will endlessly run towards you from off-screen, initiating a battle against two upon contact. This is why we're attempting the trophy here, as we're consistently forced into battles. Equip the Bloodsword on Steiner, make sure the battles progress as fast as possible in the Config menu, press R2 in battle to start auto-battle, and pause the game and activate the speed and 9999 damage boosters. Do
not activate the battle assistance booster - it forces a Trance animation to play at the beginning of every battle which considerably adds to the time - we gave Steiner the Bloodsword instead so he won't die (every enemy he hits, he will be healed for the same amount of damage). Marcus will die, but Steiner will be able to endlessly hold his own.
Unfortunately, even with auto-battle on, you'll quickly notice that you'll still need to confirm the results following each battle, so you can't just leave the game running. If you have a Turbo controller, you can set it to repeatedly press X, and that's it - you're done. Time to leave the game running and you can come back later. Otherwise, this is where Remote Play and Auto Hot Key are necessary.
So, download and install Remote Play and Auto Hot Key on your computer - they're both straightforward. Auto Hot Key will allow you use a script that makes your PS4 think you're repeatedly pressing X every second or so, and Remote Play makes that script viable since it runs on your computer, rather than on PS4. Now, open Notepad, copy and paste the following into it, and save it as an .ahk file on your desktop:
Loop
{
Send {enter down}
sleep 45
Send {enter up}
sleep 300
}
Open Remote Play, plug your PS4 controller into your computer via USB, then open the script you just saved to your desktop and click back on the Remote Play window. If you've done everything correctly, Steiner will constantly take out endlessly approaching Alexandrian soldiers without you having to do anything. The only problem here is that the script only works on the active window so you can't use your computer for anything else while this running, and obviously you can't use your PS4 either. This takes about 24 hours to do - Steiner was level 14 when I started and level 93 when I finished. Furthermore, you cannot save at any point during this method, so you really have to find some time to commit here.
I hope this was all understandable. Good luck!