I try to maximise on threats and answers vs the mirror but essentially the Jund mirror comes down to who makes the better plays - like saving
Bloodbraid Elf for maximum effect, etc. (i.e. when opponent has both cards in hand and a creature in play so whatever BBE Cascades into provides value) Personally, I like to bring in
Umezawa's Jitte,
Abrupt Decay and
Maelstrom Pulse in order to have as many answers to my opponent's threats as possible. I'll generally take out
Thoughtseize and/or
Hymn to Tourach depending on who I'm playing (as different people play Jund differently or have more sideboard options for the mirror). If you're playing against a
Punishing Fire version then some number of
Surgical Extraction should be brought in as an answer to it - as it's brutal in the mirror due to taking out every creature in the deck except
Tarmogoyf - it also wins Goyf wars.