no easy way to put this... git gud
on a first playthrough, though, best way to optimize this achievement seems to be:
1. from the first stage on keep an eye out for the secrets.
2. don't stick to the same weapons, keep varying
3. always try to do finishers with different weapons
4. always try to get kills throwing different weapons
5. A+ the level before advancing on to the next one.
Doing those will naturally give out 3/4 of the achievements.