I just recently started playing bio shock and it was really hard to kill "mr. bubbles" .
Edit: just tempered having to kill 343 :c
What enemies do you have a hard time killing ?
[spoiler]Wake up Mr. Bubbles ![/spoiler]
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Flood buddies in Combat Evolved. Shoot both arms off a combat form and you've got a little pal that follows you around. But eventually his constant staring gets annoying and you have to put him down.