On topic, after you're sent to a temple in the storyline the main story quests basically force you to get the situational achievments and the rest is through regular gameplay. Except for the already mentioned one for two undetected outposts, which is hard to miss with decent equipment.
Might want to go out of your way for the flamethrower kills and rock distraction one early since since you take all the outposts most of the enemies disappear.
Who? Who is but the form following the function of what and what I am is a man in a mask.V
Also, not that it will affect an acheivement, but there is one missable relic that you should look up. I dont want to spoil any story stuff, but eventually it will be locked off from you. Not necessary for the relic cheevo, but if you want all relics and tats and 100% on your pause screen make sure to grab it.