Glitch that happens after i change form, scrolling the stage(like in Kefka's fight)

Specific commands of battle scripts have poorly implementation and don't work correctly, except for the specific cases where they were designated. You should track the command which causes the issue. I suppose you used the command for the switch of the bosses of the Veldt Cave (“SrBehemot”). I used it in the past and it sometimes triggered strange bugs.

If it is somehow related with the transition animations, you can try different animations for the screen transitions.

My recommendation is to split the battle in two battles and change the event script to call two battles in sequence. The first battle with normal background and the second battle with the dynamic background. You can call another battle outside of the battle script.