Declare a creature as an attacker, triggering Avatar Roku, adding six {R}.
Move to your postcombat main phase. You will not lose the mana due to Electro.
Activate Aggravated Assault by paying {3}{R}{R}, causing you to untap all creatures you control and to get an additional combat and main phase after this one.
Repeat.

Infinite combat phases

Infinite red mana

Infinite untap of creatures you control

Infinite mana creatures you control can produce

Infinitely powerful red creatures you control until end of turn

Infinite damage to one opponent

Avatar Roku, Firebender

Aggravated Assault

Electro, Assaulting Battery

Activate Fire Nation Palace by paying {1}{R} and tapping it, giving Hellkite Charger firebending 4 until end of turn.
Declare Hellkite Charger as an attacker, triggering its firebending ability, its regular ability and Avatar Roku.
Resolve the Avatar Roku and firebending triggers, adding ten {R}.
Resolve the second Hellkite Charger trigger, causing you to pay {5}{R}{R} to untap all attacking creatures and to get an additional combat phase after this one.
Repeat from step 2.

Infinitely powerful creatures you control until end of turn

Hellkite Charger

Fire Nation Palace

Declare Hellkite Charger as an attacker, triggering it and Avatar Roku twice.
Resolve both Avatar Roku triggers, adding twelve {R}.
Resolve the first Hellkite Charger trigger, choosing not to pay.
Resolve the second Hellkite Charger trigger, causing you to pay {5}{R}{R} to untap all attacking creatures and to get an additional combat phase after this one.
Repeat.

Isshin, Two Heavens as One

Activate Reflection of KIki-Jiki by paying {1} and tapping it, creating a token copy of Combat Celebrant with haste.
Declare Combat Celebrant token as attackers, choosing to exert Combat Celebrant as it attacks.
When you attack, Avatar Roku and the Combat Celebrant token trigger.
Resolve the token's trigger, untapping all other creatures you control and causing you to get an additional combat phase after this one.
Resolve Avatar Roku's trigger, adding mana.
Repeat.

Combos with "Avatar Roku, Firebender"