But with my method, you upgrade it the round after you get it
Try it, see how it goes. I found the rocket launcher killed you off more than anything and the blade cannon was too annoying to upgrade as it's costly. The last area on round 9 I was crapping myself, so as soon as I entered I threw 4 turrets, 3 mines and 2 flares down at sat in the corner. The screen was lagging everywhere, then I activated those blade conveyer belt things and then all the enemies had gone. 3 - 4 slashers spawn in the final area on wave 9
I'm sure you'll get there mate, you got any of the other trophies? Oh and I got the LA Noire

today
