Death to us all

We were asked by the king to escort a cleaning crew through an abandoned prison so they could reinstate it and use it. It was very large and looked like no one had entered in about 30 years. My party gladly accepted the task at had.
The party consisted of a gunslinger, a cleric, a rogue, and a warrior. We searched every room one by one. On the first few rooms we did not find anything but, as we continued on, a giant spider jumps down and attacked the cleric with such force that he tossed in the corner unconscious  The bad news for us is that he was the only healer and no of us had any potions on hand.
I quickly draw my gun and start shooting but, before I could reload it starts charging at me. The warrior quickly runs towards the spider swinging his 6 foot sword and manages to cut 2 of the legs, which manages to slow down the creepy crawler. The rogue does what he is best at and just stealth on through ignoring the fact that we were fighting this beast mumbling something along the lines of "I think you guys can handle that..." I shoot one last time and it lands right smacked in the middle of his forehead. At last he is dead. As the cleric comes to we notice that here is green goo oozing from the spider and surrounding us to where we could not escape. As I took my last breath of air all I could think of was how we failed the mission.