Vex

Gloomist only draws after a Hold, so the unit holding the battlefield has to survive the opponent's attack first. A unit with Tank must be assigned lethal damage before other units without Tank, so a Tank can protect a more valuable holder beside it. Discipline adds two Might and replaces itself, and Defy counters a spell that costs no more than four Energy and one Power, so keep one of them available through the opponent's turn when the holder matters.

A card with Hidden costs one Power to place face down at a battlefield you control, and from the next turn on you can play it with Reaction timing while ignoring its base cost. Its choices are limited to that battlefield, and if the opponent takes the battlefield, the hidden card goes to the trash. Back Off only draws a card when played from hand. Switcheroo swaps the current Might of two units at that battlefield for the turn rather than killing anything. Tideturner is the exception to the location limit: its play ability chooses a unit you control at another location, so from Hidden it can trade places with a unit anywhere else.

Units that arrive at a battlefield during a combat there join it on their controller's side, so Hidden cards and Reaction units can change a defense after the opponent commits. Bedeviling Stranger can return an enemy unit when four or more units are at the battlefield it chooses, and Janna, Savior can be played as a Reaction to a battlefield you control to heal your units there and move up to one enemy unit from that battlefield to its base. Apathetic works in the other direction: units the opponent plays while she is at a battlefield are stunned and deal no combat damage that turn.