Went through the explorer mode a bit last night with 2 friends surprised by the challenge. We were fresh characters and they put some level 9 enemies close enough to our first couple of missions and they had giant amounts of damage reduction. I actually ran out of ammo, it was weird. It wasn't terribly fun, but once we got back on track to enemies that were on our level it went smoother with 2-5 shot kills.
Played a bit of the multiplayer. It was the control point mode and felt very Haloish till you came up against some crazy different weapons. One person had some single shot sniper rifle that was instagibbing people. Not sure if they were getting headshots or what but I was being sniped off my speeder bike almost every time I drove by them. Don't want to cry op, but I'm gonna see if I can find one of those tonight and test it out. Really weird to not have access to a gun someone else has in a game mode that is considered competitive.