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I have a Smith & Wesson MP15 AR rifle with a 6-position collapsible stock. The rifle recently rusted over a bit in my safe and now the castle nut will not unscrew. I'm using one of those AR-15 multi-tools, but even with help from a friend holding the receiver down it just won't unscrew. Any hints/suggestions?

EDIT: It's not terribly rusted, just a few orangish rust spots on it so I may just be doing something wrong. I've removed the spring and buffer and everything else already as well.

Usually you have to place the lower receiver in a vise in in order to get the castle nut off. You can buy a little block that slide into your magazine well in order to place the weapon in the vise without having to mar up the finish of the weapon.

My S&W m&p15 was staked on the castle nut. Just straighten out the marks with a punch or anchor the receiver and put some back into it.

You definately need something to hold it. Your buddy is not a solid anchoring point, sorry :)
You Should buy a Vise & A block for your lower. That's the proper way to do it... However... I have had success with another method...

I do not recommend this method at all, and for me, it fell firmly in the "I was angry, and just wanted the barrel nut off, didn't want to wait another day to go get the part, consequences be darned." category..
This can result in just plain Breaking your lower, or other parts....

...However, in the interest of full disclosure... Placing the part in a "V" In a tree in my backyard proved solid enough to let me remove the barrel nut.

Soak threads with Kroil overnight, put in vice w. vice block, and crank it off.