Comment on Difficult choice by Princesa-Daisy
There is a couple options here save peach and luigi dies and the multiverse will collapse save luigi and peach.. well meh I would save peach trade her for luigi and then go save peach
Its more easy to save Luigi because Bowser would never let Peach die.
I guess you could just throw a fire ball at bowser while he mocks mario and save them both right there besides everybody said luigi I have to think outside the 
box
boxOr you could kidnap lemmy or another koopaling and barter (I would keep lemmy)