I can't go over 45k

Hi, I've been playing Megabonk for over 200 hours, and I can't get more than 45k kills. I only reached 187k once in a random match with Clank, but now that I'm trying to get the 300k kill achievement, I can't get past 45k. I watch build videos, but I can't get past that number of kills. What can I do?