This achievement is unlocked during the 1st mission of the second Slavs campaign, "Boleslav I".
This takes place after the initial part of the mission where you hold off a siege. When the game transitions to nighttime after a cutscene and you control 3 spearmen is when this achievement starts.
Sneaking to the catapults shouldn't prove too difficult, the guards are fairly sparse, and you can often use their speech prompts to know when you can move. Use the bushes whenever you can and stay out of the patrols' torchlight.
When you reach the catapult(s) there will be a cutscene, the achievement will unlock once it is done, provided you got there undetected.