I'm STILL having trouble with the toxic gun and the way enemies take damage multiple times from the same source whilst I wish they wouldn't. I think I fixed that by increasing the invulnerability timer for giant enemies a bit and by tweaking it so all sources of damage don't trigger it. If you attack with certain gun parts equipped, you'll trigger more than one call to the 'take damage' function, since each damage has only one type (fire, ice, etc.) and you can deal multiple types per hit.
I've also increased the health of enemies a bit.
Time spent on the project so far: 420 hours
Currently working on: Continuing with the story