thnx the only advice that helped.I couldn`t return to slimes and tent as I saved after releasing prisoners and bottom exit was locked at this moment. And that was only save file.I'm not sure who this will help but this strategy got me past Grobei and could be patched in the future.
SWITCHING ELEMENTAL WEAPON TYPE WITH [V] WILL GRANT INVINCIBILITY FOR THE DURATION OF ITS ANIMATION.
Once I realized this I defeated Grobei by using a strategy of attacking him twice with [Z] to build up NRG meter then timing the animation of the Elemental switch to occur during the boss' attack. Have fun!