This is a really simple achievement, and will probably pop during your first campaign.
To unlock 'Bloody Centurion', you just have to kill 100 different enemies in the multiplayer mode. If you've not started multiplayer yet, the enemies are AI, and can be killed in anywhere from 2-7 hits.
It shouldn't take long at all.
If you want to rack it up asap, pick Jupiter as your god, earn some focus from executions, then pop focus and kill as many as you can while its active.