I think invincibility would be way too easy. You choose hard more for a reason, if you want a proper challenge. Invincibility should't be an option. For easy mode i guess would be okay. Maybe add different rewards for different difficulties: Easy - Invincibility. Normal - Infinite Grenades - Hard - Infinite Shrykull. At least with infinite shrykull you can still die, and lose the ability temporarily when you run through security lasers.
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