Wait a minute... you're renting, and you have a problem with something... you don't know how to fix it, but you're asking the landlord to fix it, even though you think he may not know how to fix it properly.
Better yet, you want to use advice found on the internet so you can watch over his shoulder as he fixes it to make sure he does it right?
What if he isn't doing it to your standards as you're watching?
I'm steering clear of this mess.
Besides, i'd have to look closely at the floor to decide if shims are going to do any good at all.