Diablo – Is your Hall of Fallen Heroes public

In cooperative play, separate tickers happen for each party member, but you don't need to have a killing blow in order to benefit.

Massacre- This bonus counts how many monsters died during one unbroken chain of your attacks. As long as you are damaging monsters constantly, with pauses of no more than 1 second, you will continue to stack the bonus. When your attacks stop connecting with monsters for about a second, the system rewards you for all the monsters you wounded, that are currently dead.

Mighty Blow- the system counts how many monsters you wounded with one attack, that happen to be dead in the following second.

Destruction- I don't think it's possible for 2 people to simultaneously damage a barrel, since they go down in 1 hit. So, no sharing this one.

If you have limited time to play and will not be online when your friends are, I recommend you play solo, at least for Normal difficulty. As Rachel mentioned, it is just as viable (until you get to very high difficulty), you will get XP and loot at the same pace, and also you may find it more fun:

• You get to play with followers (unavailable in co-op play)
• You can choose your follower and equip/spec them to complement your play style
• Followers have interesting lore and great voice-acting
• Follower AI is likely to be better than most of the random people on Battle.net
• You can complete quests at your own pace
• You can listen to dialogue and watch cinematics without people interrupting them
• Solo gameplay is arguably more tactical and strategic as opposed to mindless click-spamming

Overall I feel that Blizzard has really invested a lot into the RPG elements of this Action-RPG, and if you play solo, you may enjoy them a lot.