The Bug Butcher

This could be more easily obtained by getting the target Combo on 1 level than repeating the same level (whilst doing the combo) another 5 times (6 times in total), instead of getting it 1 time for each 6 different levels.

Note that you do not have to do it consequently, i.e. if you fail to do it just retry and it'll still work.

I did this on 2-7, it is the easiest level to get to the 50 combo, after getting the combo on this level I repeated it another 5 times, getting the combo each time and the achievement popped up.