Monsters level

[Benjamin]

Hey everyone, newish player here.

I was playing a game of munchkin the other day and came across an interesting situation. Another player was in combat with a level 4 monster which had a + 5 against dwarfs but the player was not a dwarf. He needed my help to defeat the monster, all I could do was play a card which gets rid of a level 8 monster or below. I said that technically I could not play it because against me, it is a level 9 monster even though it says level 4 on the card. He argued that I could play it because The +5 is adding to the combat level, not the monsters level.

Question is, what does make the monsters level & who was right?

Thanks, Ben