TLDR:
Use the Low-density attack on the knapsack based cryptosystem (Merkle_Hellman).

https://meashiri.github.io/ctf-writeups/posts/202306-dantectf/#adventurers-knapsack

Original writeup (https://meashiri.github.io/ctf-writeups/posts/202306-dantectf/#adventurers-knapsack).