I've done "Boom Goes the Dynamite" and "Epic Fumble" twice, but never got the achievement.

Did get the one for doing one tomb though.

I also seem unable to get "Big Game Hunter" even though I've killed probably close to 100 large animals. Also tried killing 10 in the same zone and that didn't work either.

contact Support if need be

SKTerran- I'd say they're pretty well bugged. I finished my first playthrough on normal at 100% of all areas, all tombs, etc.

Got some achievements higher up on the food chain, like No Stone Left Unturned, but for whatever reason, didn't get earlier ones like Bookworm or Historian. I also didn't get Deadeye, Former Adventurer, Get Over Here, Gunslinger, One Smart Cookie, Sharp Shooter, Tastes Like Chicken, or Widowmaker despite doing 2 additional playthroughs to specifically complete their stated conditions (though I definitely successfully pulled the numbers for Get Over Here, Sharp Shooter, and Tastes Like Chicken in my first run).

Reinstalling hasn't helped either, so I dunno what to tell ya. Still waiting to hear back from support to see if any of this is known or if I have a monkey in the system.

Good luck- will be keeping an eye on this thread in case any workarounds pop up-

Strange, I haven't noticed any achievement unlock issue so far. Is your internet connection on and galaxy runs by the time of unlocking? That's what I would check initially.

Same issue here. After I had unlocked a dozen or so achievements, I can't unlock any more. I am pretty sure, I have shot enough enemys off the lines, killed more than enough with shotgun or rifle and performed the required number of stealth kills. None of the corresponding achievements unlocked. "Down and Dirty" is the only one that came up lately and I wonder if the reason for all this is, that I abandoned my first playthrough after reaching Shanty Town to start all over again. I never performed the required action for "Down and Dirty" in my initial playthrough. Maybe the achievement system gets borked if you re-start before reaching the required number of actions for those "Do-this-n-times" achievements.

EDIT: Forget what I said. Litarally 5 minutes after this post, "Deadeye", "Equalizer" and "Get over here" unlocked in a single fight.
EDIT2: Game completed, all achievements unlocked.

Has anyone found a fix for this?
I just finished the game 100% but for some reason, the achievement for finishing all the optional tombs did not unlock.
I've went through all the tombs again, up until the chest just to double check and all of them are open.
I really don't want to play through the entire game again.

Okay, after a couple hours of trying various things, I think I know the problem.
From the research I did, sometimes the profile.dat file in your save folder doesn't record data correctly.
What you can do is make a backup of your saves folder (just in case) and delete the profile.dat file.
The game should create another one when you start it the next time.
I used a previous save I had to test this, had to play for 1 hour to get to the beach area to be able to get the final tombs, but the achievement unlocked after I completed the last tomb again.
Hopefuly this helps someone.