Simple. They could have you go to the options menu and set the kind of opponents you want - gold, silver, bronze, red/blue, or green - and have you face a number of bots at that skill level. Your partners could potentially be anyone who had the same opponent setting as you. For regular missions, your enemies would be near the objective area and, after taking them out, you'd be given a timer to show how long you have to do the objective before they get back. For dispatch missions, there'd be a timer showing how long you have until the enemies reach the objective area. And if an enemy is doing the objective, then you'd have another timer to show how long you have to stop them from completing it.
Scoring wouldn't have to be any different than how it is now.