1. Start multiplayer mission with Codex rules turned off.
2. Pick an Apothecary, level him up to lvl 2.
3. Switch to the Tactical, level him to lvl 4 with whatever weapon you like. Switch to Apothecary if you need to heal, but mind the skill cooldown after switching weapons/chars (1 minute in case of Auto-heal).
4. Summon the Servo-skull (3rd skill), injure some genestealers with your gun (stormbolter or flamethrover), let your servo-skull finish them. That'll count as your assist.
Servo-skull cooldown is 300 seconds, its duration is 120 seconds, so every 3 minutes you can get yourself some assists.

I know its lame, boring and tedious, but it is only option if you cannot find battle-brother willing to help you.

Thanks to strider_9000 for the idea.