Alright, on top of the Shadowbound=>Surrender=>Llavanya's Melody=->Soul Link Spam exploit, I found another big exploit in v3.1.
There is a path (all submissive responses except for trap options) where you can continue to fight Katrina after getting the Starlight Seal, although you are reduced to LVL1 when you enter her realm. You CAN BEAT HER.
While outside her realm at full strength, you can use Shadowbound and stack Rage of Llavanya OUTSIDE OF COMBAT. The effects will apply after you enter Katrina's realm and start the fight. With the Golem Sword equipped, I was able to defeat her, gain 500XP (+6 Foreign Soul Shards) and advance up to LVL3.
When I left Katrina's realm, I had gained 2 levels toward my full strength stats. When I return, I retain the levels I gained by earning XP.
After grinding with Katrina up to LVL 5, I noticed that leaving her realm also resets the XP requirements to gain levels and I had to defeat her multiple times without leaving her realm to gain enough XP to level up.
At LVL 10 I defeated Katrina witout potions or preloading my ATK buff. I'm currently at LVL11 in Katrina's Realm (LVL36: MAX 2200HP and 440MP outside Katrina's realm) and I'm certain that (with patience) I'll soon reach the point where Llavanya's Melody will restore 150 MP, which will allow me to spam the exploit I mentioned at the beginning of this post and eventually max out my level without any save editor cheats.
I like games like this best when I have no choice but to get my ass handed to me

. If
Dekarous is just doing this for fun, no problem, but if he's aspiring to be a legit developer, these are the kind of things that need to be fixed to create a tight gameplay experience.