If Mardu Siegebreaker leaves the battlefield before its "enters" ability resolves, the target creature won't be exiled
While it has hexproof from artifacts, creatures, and enchantments, one of the most notable features of Nevinyrral is that, upon entering the battlefield, it creates a 2/2 zombie token for each creature that had died that turn
How much do they need that ability
Extra-wide interior with space for Outer Sleeves (Over Sleeves), Mini Snaps or card dividers alongside your deck
You can have it sitting on your Bench while your other Pokemon start punching at your opponent
The trials will see High Tide and Prophetic Wisdom become legal until the next banning and restriction announcement by Wizards, and until then developers will examine and take feedback as to whether or not the cards should be re-banned or enter legality for Pauper going forward