at line 687 of bullets.c:
There is handled the hit at robots. There can be a random function that get a
float number. If this number is lower or equal to tux critical rate, instead of
using the normal method, us use a copy of this, but this time, multiply the
damage numbers in, by example, 1.2 (20% more!).

To bots have a critical rate, it would be difficult, but the "hit tux" is at
line 610 of the same file(src/bullet.c).

This implement a critical hit, the only problem is that tux critical hit would
need be manipuled by other functions, like