I used to play Halo by myself a lot, but I definitely played with friends. With Halo 5, I try not to play Arena at all, especially by myself. If it's Action Sack then maybe, but a playlist like BTB I'm usually the powerhouse or the main slayer of the team. The past two days I've been averaging something like 20 - 25 maybe 30 kills a match minus the last match I played last night. Not really impressive when you think about it; I searched with two friends whose MMR values are much lower than mine, not to mention a lot of the players who play BTB are a little lower than average at best, at least from my experience.
Sometimes I'll play Warzone Assault and Warzone by myself but recently I've been staying away from those playlists. A lot of farming and one-sidedness going on more recently. I will search Firefight regardless, however.