Ok, so every time I use a crossbow or, in rare cases, a cleaver from Tinker's Construct and successfully killing mobs, like the moment they've been slayed, the game crashes... Something interesting to note is that this issue used to not occur so, I don't know what's going on... I reported this issue to the Tinkers' Construct Issue page and, they said that the main issue is from Levels. From what I can discern, it seems like Levels is trying up my TiC Weapons when it's actually incompatible with it, therefore causing a crash. I think a simple hotfix should do the trick, but make sure it completely ignores TiC weapons because, modifiers correctly serves that purpose. Although, of course, it's from an older, MC version (1.7.10) so if you can't fix it, that sucks but... oh well...

Here's the crash report...
https://pastebin.com/gp81cAfP