What does the Slime-Proof Poncho do?
 

Rather than get to the back story of the situation that caused my confusion to arise, I will simply ask: "How EXACTLY does the 'Slime-Proof Poncho' card work? Can ichors that would modify the monster, not the wearer, still be used in combat? ex. I am wearing the poncho and my friend tries to substitute the monster I'm fighting with the 'Tricky Ichor.' Can this actually be done?"

Re: What does the Slime-Proof Poncho do?
 

The Slime-Proof Poncho prevents you from being affected by an Ichor unless you play it on yourself. Since an Ichor that helps the Monster side affects you negatively, it wouldn't affect you at all if you're wearing the Slime-Proof Poncho. In the other example you give: No, the other player could not have played the Tricky Ichor and expect to be able to do anything involving a combat you were in with that card if you were wearing the Slime-Proof Poncho.