The rules say to add your combat strength to theirs and in a separate section it says that your combat strength is your level plus weapons etc.
In our first few games this essentially meant that every monster was easily beaten so we were wondering if maybe it was just your level you add to them and any enhancement cards.
Yes, the rules do specifically say
Combat strength is the total of Level plus all modifiers - positive or negative - given by items and other cards.
So the helping player adds all their bonuses to yours. Remember also to count situational modifiers that are brought about by the new helper eg. a monster who is +4 against Elves would gain a +4 bonus if the person you brought in to help was an Elf but you weren't.
Just remember that Munchkin is a competitive game and it is up to you whether you assist or not (unless a specific card says otherwise). It isn't always in your (or your fellow player's) best interests to agree to help, thus forcing the player in the combat to fight alone or run away.
Your combat strength is your level plus any bonus from your equipment or any items you might use, which definitely will make some monsters much easier to defeat than otherwise.
