"Hmmm, maybe this was a design error on my part, mixing deep thinking in a shooter game, I'm not sure?"
Not at all, sounds like a classic final boss achilles heel to me. I haven't played the game so I don't know how blatant the clue to defeat the boss is.
Before each boss shows up, a short message shows up for about 2-3 seconds, telling the player what that boss is about. In this case, it says: Grows When Shot.
The only thing is that this is not the final boss. This is the boss on level 70. Next one where you need to figure out the message meaning is on level 120, and there are even more bosses after that.
Not at all, sounds like a classic final boss achilles heel to me. I haven't played the game so I don't know how blatant the clue to defeat the boss is.