If there's a gigantic showdown that he loses, he can then blame Parliament for Brexit failing & sail on as PM.
What will he do to achieve that? Absolutely anything.
If it includes collapsing Brexit but that keeps him in power until 2022, he wouldn't even blink.
Either he gets to no deal Brexit, which would make the Brexit Party redundant and bring ex BP voters back to the Tories...
Or Brexit is cancelled but it's not his fault.