I am far from pro. But if you are playing against a team with no area of effect and your teammate tells you he's going full minion, there are two basic things you need to do. Do not die. And cap and lock flags. The second includes protecting the portal and gold flags with locks if necessary. And if you can succeed in this, then the minion player can win the game in 17 minutes and 46 seconds. If you fail at either of these issues, then you can cause a loss.
Your minion player will not be buying citadel upgrades, he will be striving for forest band, gloves of fel dur and for seige demolishers. Once this is achieved, the game is basically over, so long as giants aren't storming your own citadel.
Rant over.