There are are some known exploits that enable to unlock all upgrades, agony bolts and melee upgrades in one playthrough. I was doing all the steps explained in this trophy guide:
Please - read unstoppable arsenal trophy guide for example. This exploit still works (you only need to uninstall the game and then install it again without installing a patch). This exploit is fully legal.
As for the 200 and 400 kills trophy - knoef_NL - mentioned it, that there is a known glitch that prevents from pop'ing that trophy. I think I did that by installing and unisnstalling the game again, without loading a patch - I triggered both trophies - they popped for me after i loaded save file and killed first opponent (one after another). Both of those trophies are really easy to obtain and you simply can not miss them if you finish the game (which I of course did). I really don't see the reason to cheat for those trophies.