I think its still to imba, there could be exploits with it. ur intention is to prevent fradz getting damage right? so why dont u change it into something like a kind of manashield, that triggers when getting attacked and can absorb a certain amount of phyiscal and magic damage.
e.g:
lvl 1 10% absorbing 500 damage, lasts 5 sek
lvl 2 10% absorbing 750 damage, lasts 5 sek
or
u can just create another passive ability that gives him a chance do dodge spells and physical damage.
i just think complete invulnerable is too imba. and sometimes u want others to back off and heal so u have to use fradz as tank and lure the creeps away, but it wouldnt work if he is invulnerable.
but depends on ur intention with this spell i guess