Card Spotlight
A 3-mana indestructible God that puts a +1/+1 counter on a creature or enchantment you control whenever you gain life, and gives another creature lifelink for {1}{W}. Give Walking Ballista lifelink and each ping puts its own counter back, for unlimited damage.
Commander Spotlight
Heliod, Sun-Crowned ↗
A 3-mana indestructible God that puts a +1/+1 counter on a creature or enchantment you control whenever you gain life, and gives another creature lifelink for {1}{W}. Give Walking Ballista lifelink and each ping puts its own counter back, for unlimited damage.
See Heliod, Sun-Crowned's full stats and printings →
Heliod is a mono-white commander for lifegain and +1/+1 counters, and the combo with Walking Ballista is always one card away because Heliod sits in the command zone. Indestructible means destroy effects and damage don't remove it, though exile and bounce do. It is rarely a creature in practice: it needs five devotion to white before it can attack.
In the 99 it fits any white lifegain or counters deck. It does useful work every time you gain life, and it carries the Ballista combo as a finish you can tutor for. The combo is less reliable here than with Heliod in the command zone, since you need to draw both pieces.
Walking Ballista ↗
The classic partner. With lifelink from Heliod, each counter it removes deals 1 damage and comes back, so the damage is unlimited.
Spike Feeder ↗
Removes a counter to gain 2 life, and Heliod puts the counter back. Unlimited life in any deck that also has green.
Soul Warden ↗
A 1-mana life gain trigger for each other creature that enters, which Heliod turns into a steady stream of counters.
Archangel of Thune ↗
Puts a +1/+1 counter on each creature you control whenever you gain life. During the Ballista loop, every ping grows your whole board as well.
Heliod is mid-priced. It has been reprinted in Commander Masters and in Secret Lair, which keeps the standard printing within reach. Walking Ballista has also been reprinted several times. Buy Heliod for a lifegain or counters deck first and treat the combo as part of what you are paying for.
Pay {1}{W} to give Walking Ballista lifelink. Remove a +1/+1 counter from the Ballista to deal 1 damage to any target. Lifelink means you gain 1 life, Heliod's trigger puts a +1/+1 counter on the Ballista, and you are back where you started with one more point of damage dealt. Repeat until every opponent is dead. The Ballista needs at least one counter to start, so cast it with X of 1 or more.
Seven in total: {2}{W} for Heliod, {2} for Walking Ballista with X equal to 1, and {1}{W} for the lifelink activation. The pieces don't have to come down on the same turn, and the lifelink ability can be activated at instant speed, so a common line is to have both on the battlefield and activate it at the end of an opponent's turn.
No. Heliod and Walking Ballista are both legal in Commander and neither is on the Game Changers list. The bracket question comes from the combo instead: Wizards' bracket rules keep two-card infinite combos out of Brackets 1 and 2 and early-game ones out of Bracket 3. With Heliod as your commander the pair can be ready on turn four, so say so at Rule 0 before you sit down at an Upgraded table.