There are 8 bomb missions in total. You do NOT need to complete the bomb challenges for this achievement.
Make your way to Piri Reis in the imperial district.
In this room there are four paintings on the wall. Walk up to one of the paintings and press b to play the mission.
Each painting has two
missions that need to be completed. You will know you have finished both missions for a painting when that painting no longer flashes (notice in the picture only the painting on the right is flashing, the rest have been completed)
Once the eighth mission is complete the achievement should immediately pop.