a last resort
The only remaining option that may help one or be of any use after all other options have been exhausted. I didn't really want to ask Mr. Stadler to write my college recommendation, but he's my last resort at this point.
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.
(as) a last reˈsorta thing you decide to do when everything else has failed: Nobody wanted to lend me the money. As a last resort I asked my brother-in-law, and luckily he was able to help me.
Farlex Partner Idioms Dictionary © Farlex 2017