Activate Heartless Hidetsugu by tapping it, dealing damage to each opponent equal to half their life total, rounded down, times 2, and dealing no damage to you due to Pariah's Shield.

Near-infinite damage

Heartless Hidetsugu

Collective Inferno

Pariah's Shield

Cast Martyrdom by paying {1}{W}{W}, targeting Phyrexian Vindicator.
Activate Phyrexian Vindicator by paying {0}, targeting a creature.
Attempt to deal damage to the targeted creature. The damage will then be increased due to Collective Inferno, then redirected to Phyrexian Vindicator, which is then redirected back to the creature that was originally targeted.
Holding priority, activate Phyrexian Vindicator by paying {0}, targeting the creature. 
Repeat step 4 until all damage will be redirected to Phyrexian Vindicator.
Repeat from step 3.

Infinite damage to one opponent

Phyrexian Vindicator

Martyrdom

Activate Deathless Angel twice by paying {W}{W}{W}{W}, giving two creatures an opponent controls indestructible until end of turn.
Deal noncombat damage to one of the indestructible creatures, dealing an increased amount of damage instead due to Collective Inferno.
Toralf triggers, dealing an amount of damage over the excess damage to the second indestructible creature due to Collective Inferno.
Toralf triggers, dealing an amount of damage over the excess damage to the first indestructible creature due to Collective Inferno.
Repeat from step 3 an arbitrarily large number of times.
Resolve a Toralf trigger, dealing infinite damage to any target.

Toralf, God of Fury // Toralf's Hammer

Deathless Angel

Give Walking Ballista lifelink, if needed.
If Walking Ballista has less than two +1/+1 counters on it, activate Unspeakable Symbol by paying 3 life, putting a +1/+1 counter on Walking Ballista.
Activate Walking Ballista's second ability by removing a +1/+1 counter from it, dealing 4 damage to an opponent or a permanent an opponent controls, and causing you to gain 4 life.
Repeat from step 2.

Infinite damage

Infinite lifegain

Infinite lifegain triggers

Infinite +1/+1 counters on creatures you control

Walking Ballista

Unspeakable Symbol

Tor Wauki the Younger

Activate Anti-Venom using Walking Ballista's ability by removing a +1/+1 counter from it, targeting itself and putting two +1/+1 counters on it due to its replacement effect and Collective Inferno.
Activate Anti-Venom using Walking Ballista's ability by removing a +1/+1 counter from it, dealing 1 damage to any target.
Repeat.

Infinite +1/+1 counters on a creature

Anti-Venom, Horrifying Healer

Agatha's Soul Cauldron

Activate Deathbringer Thoctar by removing a +1/+1 counter from it, putting two +1/+1 counters on it due to Vigor.
Activate Deathbringer Thoctar by removing a +1/+1 counter from it, dealing 2 damage to any target.
Repeat as desired.

Combos with "Collective Inferno"