Single player is ok, but the game is really meant for group action, the more you have in the group the more fun.
Example - If your solo the missions will spawn about 4 mobs in a wave, if theres 2 of you, it will spawn 6 mobs, 3 players 8 mobs, these arent actual numbers, just a rough guess, I dont usually have time to sit & count them.
In a group you can also set up differently to solo, you arent going to need 4 with a scan, 2 at most, then one can have a sticky bomb or seeker mine, & the other can use a shield, or whatever you feel like.