Walking Ballista

Bracket

• B1 Exhibition — Not in the same deck as its two-card combo partners: Heliod, Sun-Crowned, Archangel of Thune, Mikaeus, the Unhallowed, Cleric Class, Light of Promise, The Destined White Mage, Sunbond, Spider-Man, Peter Parker, Enduring Renewal, Urban Daggertooth, Heroic Feast, Bellowing Aegisaur, Vraska, Swarm's Eminence, Mephidross Vampire
• B2 Core — Not in the same deck as its two-card combo partners: Heliod, Sun-Crowned, Archangel of Thune, Mikaeus, the Unhallowed, Cleric Class, Light of Promise, The Destined White Mage, Sunbond, Spider-Man, Peter Parker, Enduring Renewal, Urban Daggertooth, Heroic Feast, Bellowing Aegisaur, Vraska, Swarm's Eminence, Mephidross Vampire
• B3 Upgraded — Not in the same deck as its early two-card combo partners: Heliod, Sun-Crowned, Archangel of Thune, Cleric Class, Light of Promise, The Destined White Mage, Sunbond, Spider-Man, Peter Parker, Heroic Feast, Vraska, Swarm's Eminence, Mephidross Vampire
• B4 Optimized — allowed: No restriction beyond the banned list
• B5 cEDH — allowed: No restriction beyond the banned list

Why

• Two-card combo with Heliod, Sun-Crowned, Archangel of Thune, Mikaeus, the Unhallowed, Cleric Class, Light of Promise, The Destined White Mage, Sunbond, Spider-Man, Peter Parker, Enduring Renewal, Urban Daggertooth, Heroic Feast, Bellowing Aegisaur, Vraska, Swarm's Eminence, Mephidross Vampire (early with Heliod, Sun-Crowned, Archangel of Thune, Cleric Class, Light of Promise, The Destined White Mage, Sunbond, Spider-Man, Peter Parker, Heroic Feast, Vraska, Swarm's Eminence, Mephidross Vampire)

How Lotus Vault estimates a bracket

Bracket: Lotus Vault analysis, with Commander Spellbook flags

Combos

381 known combos, 14 of them two-card.

• Heliod, Sun-Crowned + Walking Ballista
  Produces: Infinite damage, Infinite lifegain, Infinite lifegain triggers
  View on Commander Spellbook
• Gatta and Luzzu + Hardened Scales + Walking Ballista
  Produces: Infinite damage, Infinite +1/+1 counters on a creature
  View on Commander Spellbook
• Archangel of Thune + Walking Ballista
  Produces: Infinite damage, Infinite lifegain, Infinite lifegain triggers
  View on Commander Spellbook

Combos: Commander Spellbook

Rulings

• 2020-08-07 · Wizards of the Coast
  If Walking Ballista has been dealt damage or had its toughness reduced by an effect, this limits how many times you'll be able to remove +1/+1 counters from it in a single turn. For example, if it has three +1/+1 counters on it and has been dealt 1 damage this turn, it will be destroyed immediately after you activate the ability a second time and you won't be able to activate it a third time.
• 2020-08-07 · Wizards of the Coast
  A casting cost of {X}{X} means that you pay twice X. If you want X to be 3, you pay {6} to cast Walking Ballista.

Rulings: Scryfall · Wizards of the Coast