I didn't realise you could exchange weaponry / items.
Re: the game's difficulty, sadly I was railroaded into watching The Rock on TV last night. Dreadful film, but one line had added resonance for me.
"The moment you don't respect this, it kills you."
its quite easy to do, just meet up with someone on a level which you havnt completed yet. remember they have to be 20 levels higher or less than yourself otherwise you wont be able to summon them. once you meet up drop your weapon, item etc on the floor and let the other person pick it up. you can unlock quite a few trophys this way!!