Doing solo, this is fairly straightforward. With the monster kills +X experience property, this will add X experience points per monster killed.
How does this work in multi play however? Do I have to kill the monsters myself, or is "monster kill" meant to be global (i.e. does it give me the experience bonus if someone else lands the killing blow)?
It works in multiplayer exactly the same way it works in single player. If you are within the "screen and a half" range for getting xp, it works as if you had killed the monster yourself.
I played through all of normal difficulty in a party of 3, and the person who had the most +xp per kill was always the one who leveled first. Most of the time it was me, and I wasn't getting most of the killing blows based on the number of monster streaks that the other two were reporting.
