*a small hint*
The other prisoner is useful if you find a way to deal with the sentry.
*a slightly larger hint*
There are two different ways to deal with the sentry. Both involve throwing something into the sentry room from outside it.
I already threw something into the sentry room to lure in the monster, but when I try to go inside afterwards, I still get turned into swiss cheese :(
*next hint below*
there is a way to make the monster grow, and the bigger monster is able to destroy the sentry. (But this is the complicated solution. The simpler one is that there is a specific item that can stop the sentry by drawing its fire...)
