Co-op campaign and Drain Cleaner help?

I’d like to get the campaign over with one run on easy. Basically to get drain cleaner you have to kill 20 enemy units with a continuous covenant leader power. I know that you guys will automatically say this is boosting if it’s for an achievement. But this can be done with another person in a private match. (which does not affect your score online at all)

Here’s my reference: skip to 3:00

GT: Mr Fuzy Unibrow