Invulnerability:

Go into your Blood Omen 2 directory (i.e. D:\games\blood omen 2 .... etc) 
Get into Data folder and locate game.erg file. You can open it with notepad. 
Within game.erg locate the following line:
"-BO2 Kain Debug Flags\kain's invulnerable"=1 

All you need to do is to delete "-" so the line would read:
"BO2 Kain Debug Flags\kain's invulnerable"=1