It was pretty buggy for me even after they apparently removed one of the fireworks you need to fire. I am positive I did both of them on my first playthrough and I only managed to trigger it by replaying the levels a bunch of times (not completely) until it worked. I was stuck at 1/2 in the achievement counter the whole time even though I triggered both of them multiple times.
Unfortunately I am still not sure why it finally triggered (I eventually got it on level 3), but I wanted to write a guide just in case just to say keep trying a bunch of times. Luckily the needed fireworks are not too far into their levels.
They are at:
- Level 3: Fireworks on the roof, keep going up on the left side of the level until you are on the top roof then go to the right.
- Level 5: On the first big space with a lot of people you encounter on the right side of the singing woman (in a basket).
I don't think it's needed any more as they made changes to some achievements apparently to try and fix the bugs, but I will mention the 3rd firework just in case. It's in level 9 (the bazaar) on top of a building that is called "Giant Cave Market". You have to go pretty far into the level to get there, it's actually relatively close to the end. You have to pass a bridge over the road from the right side to get there.
Hopefully this helps.