WET

For this Achievement I recommendd to start Level 5 and go for the multiplier around the area 'til you get 5X , keep killing the enemies without lossing the multiplier and you should get the achievement.

Btw. good place to go for the 200 kills (with the different weapons)

Easy achievement got it on my second play through ... Like the achievement reads all you is kill 100 enemies while having a x5 points multiplier ... I would recommend playing on normal ... The best way to get a x5 multiplier is to use the dive & slide combo, make sure your not just standing still killing enemies keep using combos ... Collect the bonus multiplier circles and leave the enemy doors open until they stop coming out ... only use the bonus circles to get a x5 multiplier or if you drop to a x4 and need a little boost.

I found it easiest to obtain on Points Mode which is available after you complete the game.
I started on the first mission, run through to the first arena, got my 5x bonus and didn't shut the doors until they stopped spawning, you can get a good 50 or so kills for keeping the doors open. I then proceeded to the final arena I did however get a few 5x bonus kills on the Basket ball court but it's not necessary to do so.
Once I got to the final arena I used the poles to swing for the bonus' once I had my 5x I did the same as I did in the first arena, let them spawn until they spawn no more, I was able to get the achievement and still have plenty to kill if I still needed it. :)
Good luck! :D

Note: When playing in Points Mode you have all of Rubi's Ability making it easier to keep a higher bonus. :)