This achievement is actually really easy. First you'll need a weapon which has the bloodlust power. I bought mine but you can also pick one up from a dead enemy. A melee weapon like a sword is better than a ranged weapon and the lower the weapon's attack power, the better since you can land more hits before killing your opponent. If you find ANY item with bloodlust, you can try transferring the power to a good weapon like a sword using synthesis.
Once you have a suitable weapon, warp to the first region of the game and head up the path. When you come to a group of enemies, start attacking. You can't let too much time go between attacks or you'll break your combo so try to attack in the direction of enemies instead of running over to them and attacking. I'm not sure if taking damage always breaks your combo but many hits will pause you long enough to break it. You might want to head past the regular enemies to get to any wizards and archers in the back. Once they are dead you can go for your achievement on the grunts. Those archers and wizards have an annoying habit of breaking your combo just before 20. Be sure to watch your health if you are still low level when you try this. Make sure you have some red potions mapped to your bumpers for a quick fix.
I think I was level 8 when I got this achievement. I was using the elf girl equipped with an attack power 3 sword. I kept it equipped for a while and there were a lot of places where I got my combos into the 30s in those early levels. The hardest part was finding a suitable sword.