Battle Factory trouble

Been having trouble trying to reach the Frontier Brain in the Battle Factory- when I first started I could easily make it to him. But now it has been consistentely harder trying to last into the third round of matches. I've been trying my best to pick pokemon with better type matchups and bulkier stats, focusing on knocking out enemies quickly rather than draining/increasing stats and statuses.
But to no avail- can anyone suggest a solid plan for picking rentals and how to best use them?