You get a bonus challenge if you survive 3 waves in a row without dying, you can get downed, on the fourth wave you will get the challenge pop up (they vary, some are weapon specific and some are just kill 10 people in 30 seconds). You have to complete the challenge during the wave you get it, once complete you'll get two boxes drop with a $ symbol, and that is one more towards your achievement.
When in horde, they sometimes give bonus rounds such as kill 6/7 using chainsword on lancer or the bayonett on classic lancer. whn u complete these u will get a box its marked with a star somewhere on the map once uve opened some of these ul get the achievement.
