Should be an event.
onBotSpawn > Bot > setDifficulty > [option]
0-Dumb as a brick, won't shoot anyone. (optional)
1-Noobish, will shoot, but not much, inaccurate.
2-Normal, same as they are now.
3-Skilled, shoots a bit more, detects players better.
4-Aimbot, shoots a bunch, always hitting everything. (also optional)
Edit: Also maybe types, like different ways they do things.
Bullethell, shoots every chance it gets.
Sparing, shoots once every so often, compared to the burst it usually does.
AND, of course, a random option.