You have a way to give Boros Reckoner lifelink.
You have a way to deal damage to Boros Reckoner.

Give Boros Reckoner lifelink.
Cast Boros Charm by paying , giving permanents you control indestructible until end of turn.
Deal damage to Boros Reckoner.
Boros Reckoner triggers, dealing that much damage to itself and causing you to gain that much life.
Repeat step 4 any number of times.

Boros Reckoner has indestructible.

Deal damage to Boros Reckoner.
Boros Reckoner and Blazing Sunsteel trigger.
Resolve the Boros Reckoner trigger, dealing damage to any target.
Resolve the Blazing Sunsteel trigger, dealing damage to Boros Reckoner.
Repeat from step 2.

Boros Reckoner has indestructible.

Deal damage to Boros Reckoner.
Boros Reckoner triggers twice. Resolve the first trigger, dealing damage to any target.
Resolve the second Boros Reckoner trigger, dealing damage to itself.
Repeat from step 2.

You can use Wayta's fight ability to deal damage if desired.

Deal damage to Boros Reckoner
Fiendlash and Boros Reckoner trigger.
Resolve the Boros Reckoner trigger, dealing damage to any target
Resolve the Boros Reckoner trigger, dealing at least 1 damage to itself.
Repeat from step 2.

Boros Reckoner has indestructible.

Deal damage to Boros Reckoner
Donna Noble and Boros Reckoner trigger.
Resolve the Donna Noble trigger, dealing that much damage to an opponent.
Resolve the Boros Reckoner trigger, dealing that much damage to itself.
Repeat from step 2.

Boros Reckoner has indestructible.

Deal damage to Boros Reckoner.
Boros Reckoner triggers, dealing damage to any target
Guilty Conscience triggers, dealing damage to Boros Reckoner.
Repeat from step 2.

You have a way to deal exactly 1 or 2 damage to Boros Reckoner.

Deal exactly 1 or 2 damage to Boros Reckoner.
Rite of Passage and Boros Reckoner trigger twice.
Resolve both Rite of Passage triggers, putting two +1/+1 counters on Boros Reckoner.
Resolve the first Boros Recknoer trigger, dealing damage equal to the damage dealt to it to any target.
Resolve the second Boros Reckoner trigger, dealing damage equal to the damage dealt to it to itself.
Repeat from step 2.

Shielded by Faith attached to Boros Reckoner.
You have a way to give Boros Reckoner lifelink.
You have a way to deal damage to Boros Reckoner.

Give Boros Reckoner lifelink.
Deal damage to Boros Reckoner.
Boros Reckoner triggers, dealing that much damage to itself and causing you to gain that much life.
Repeat step 4 any number of times.

You have a way to give Boros Reckoner lifelink.
You have a way to deal damage to Boros Reckoner.

Give Boros Reckoner lifelink.
Deal damage to Boros Reckoner.
Boros Reckoner triggers, dealing that much damage to itself and causing you to gain that much life.
Repeat step 3 any number of times.

Indestructibility attached to Boros Reckoner.
You have a way to give Boros Reckoner lifelink.
You have a way to deal damage to Boros Reckoner.

Give Boros Reckoner lifelink.
Deal damage to Boros Reckoner.
Boros Reckoner triggers, dealing that much damage to itself and causing you to gain that much life.
Repeat step 4 any number of times.

Pariah attached to Boros Reckoner.

Cast Volcano Hellion by paying .
Volcano Hellion enters the battlefield, dealing an arbitrarily large amount of damage to Boros Reckoner.
The damage Volcano Hellion would deal to you is instead dealt to Boros Reckoner due to Pariah.
Boros Reckoner triggers, dealing an arbitarily large amount of damage to target opponent.

Pariah's Shield attached to Boros Reckoner.

Cast Volcano Hellion by paying .
Volcano Hellion enters the battlefield, dealing an arbitrarily large amount of damage to Boros Reckoner.
The damage Volcano Hellion would deal to you is instead dealt to Boros Reckoner due to Pariah's Shield.
Boros Reckoner triggers, dealing an arbitarily large amount of damage to target opponent.

Dawnsire has ten or more charge counters on it.

Declare any creature as an attacker. Dawnsire triggers, dealing 100 damage to Boros Reckoner.
Boros Reckoner triggers, dealing 100 damage to any target

You have a way to give Boros Reckoner lifelink.
You have a way to deal damage to Boros Reckoner.

Give Boros Reckoner lifelink.
Deal damage to Boros Reckoner.
Boros Reckoner triggers, dealing that much damage to itself and causing you to gain that much life.
Repeat step 4 any number of times.

You have a way to give Boros Reckoner indestructible.

Give Boros Reckoner indestructible, if needed.
Cast Chain of Plasma by paying , dealing 3 damage to Boros Reckoner and causing you to discard a card and copy Chain of Plasma targeting Boros Reckoner.
Taii Wakeen and Boros Reckoner trigger.
Resolve the Taii Wakeen trigger, causing you to draw a card.
Resolve the Boros Reckoner trigger, dealing 3 damage to any target.
Resolve the Chain of Plasma copy, dealing 3 damage to Boros Reckoner and causing you to discard a card and copy Chain of Plasma targeting Boros Reckoner.
Repeat from step 3.

Soul Link attached to Boros Reckoner.
You have a way to give Boros Reckoner indestructible.
You have a way to deal damage to Boros Reckoner.

Give Boros Reckoner indestructible, and then deal damage to Boros Reckoner.
Boros Reckoner and Soul Link's second ability trigger.
Resolve the Soul Link trigger, causing you to gain that much life.
Resolve the Boros Reckoner trigger, dealing that much damage to itself.
Boros Reckoner and Soul Link's first and second abilities trigger.
Resolve both Soul Link triggers, causing you to gain life equal to twice the damage dealt.
Repeat from step 4.

Cast Angel's Grace by paying .
Cast Volcano Hellion by paying .
Volcano Hellion enters the battlefield, dealing an arbitrarily large amount of damage to Boros Reckoner and you, with Angel's Grace causing your life total to become one instead.
Boros Reckoner triggers, dealing an arbitarily large amount of damage to target opponent.

Gift of Doom attached to Boros Reckoner.
You have a way to give Boros Reckoner lifelink.
You have a way to deal damage to Boros Reckoner.

Give Boros Reckoner lifelink.
Deal damage to Boros Reckoner.
Boros Reckoner triggers, dealing that much damage to itself and causing you to gain that much life.
Repeat step 4 any number of times.

Tree of Perdition does not have summoning sickness.
You have enough mana available to cast Blasphemous Act.

Activate Tree of Perdition, exchanging target opponent's life total with Tree of Perdition's toughness.
Cast Blasphemous Act, dealing thirteen damage to all creatures.
Boros Reckoner triggers, dealing thirteen damage to target opponent.

Your life total is at most 5.
You have a way to give Boros Reckoner lifelink.
You have a way to deal damage to Boros Reckoner.

Give Boros Reckoner lifelink.
Cast Break of Day by paying , giving creatures you control indestructible and +1/+1 until end of turn.
Deal damage to Boros Reckoner.
Boros Reckoner triggers, dealing that much damage to itself and causing you to gain that much life.
Repeat step 4 any number of times.

Tree of Perdition does not have summoning sickness.

Activate Tree of Perdition, exchanging target opponent's life total with Tree of Perdition's toughness.
Cast Star of Extinction for , destroying target land and dealing 20 damage to all creatures and planeswalkers.
Boros Reckoner triggers, dealing 20 damage to target opponent.

Cast Volcano Hellion by paying .
Volcano Hellion enters the battlefield, dealing an arbitrarily large amount of damage to Boros Reckoner and you.
You do not lose the game due to having zero or less life due to Platinum Angel.
Boros Reckoner triggers, dealing an arbitarily large amount of damage to target opponent.

You have a way to deal exactly 1 damage to Boros Reckoner.
You have a way to give Boros Reckoner indestructible.

Give Boros Reckoner indestructible, if needed, and then deal exactly 1 damage to Boros Reckoner.
Boros Reckoner and Ghyrson Starn trigger.
Resolve the Ghyrson Starn trigger, dealing 2 damage to Boros Reckoner.
Boros Reckoner triggers, dealing 2 damage to any target.
Resolve the Boros Reckoner trigger from step 2, dealing 1 damage to itself.
Repeat from step 2.

Tree of Perdition does not have summoning sickness.

Activate Tree of Perdition, exchanging target opponent's life total with Tree of Perdition's toughness.
Cast Shivan Meteor for , dealing thirteen damage to Boros Reckoner.
Boros Reckoner triggers, dealing 20 damage to target opponent.

Cast Volcano Hellion by paying .
Volcano Hellion enters the battlefield, dealing an arbitrarily large amount of damage to Boros Reckoner and you.
Your life total does not change due to Platinum Emperion.
Boros Reckoner triggers, dealing an arbitarily large amount of damage to target opponent.

Tree of Perdition does not have summoning sickness.

Activate Tree of Perdition, exchanging target opponent's life total with Tree of Perdition's toughness.
Cast Into the Maw of Hell for , destroying target land and dealing thirteen damage to Boros Reckoner.
Boros Reckoner triggers, dealing 20 damage to target opponent.