Walking Ballista [Aether Revolt] Desert you can't be attacked except

you can't be attacked except by creatures with flying and/or islandwalk

Set: New Phyrexia Type: Artifact Creature — Phyrexian Horror Rarity: Rare Cost: {2} {U/P}: Change a target of target spell or ability to Spellskite

Set: From the Vault: Relics Type: Artifact Rarity: Mythic Cost: {0} Sacrifice a land: You gain 2 life

Then exile it

" —Seton

Walking Ballista [Aether Revolt] Desert you can't be attacked exceptSet: Aether Revolt Type: Artifact Creature Construct Rarity: Rare Cost: {X}{X} Walking Ballista enters the battlefield with X +1 +1 counters on it. {4}: Put a +1 +1 counter on Walking Ballista. Remove a +1 +1 counter from Walking Ballista: It deals 1 damage to any target.