I agree. Said "guest" could have either insane defense or evasion, or a barrier like some enemies do.
Sure, that means you have an invincible "party member", adding that other enemies wouldn't target it. Then again, sometimes, "guest" party members in games really are invincible, but on the trade-off that they're not THAT good in combat, and/or if the main party is defeated it's still game over even if the guest is still battle-capable.
So perhaps the same could be done here.