At the beginning of your combat phase, Helm of the Host triggers, creating a nonlegendary token copy of Éomer with haste.
Declare Stangg and the additional legendary creature as attackers.
Stangg triggers, creating a 3/4 Stangg Twin creature token that is tapped and attacking.
Holding priority, kill the additional legendary creature.
When the creature dies, each Éomer that has not triggered this turn triggers, causing you to untap all creatures you control and to get at least one additional combat phase after this one.
Move to your next combat phase.
At the beginning of your combat phase, Helm of the Host triggers, creating a nonlegendary token copy of Éomer with haste.
Declare Stangg as an attacker.
Stangg triggers, creating a 3/4 Stangg Twin creature token that is tapped and attacking.
Put either Stangg Twin token into your graveyard due to the legend rule.
Repeat from step 5.

Infinite combat phases

Infinite death triggers

Infinite creature ETB

Infinite creature LTB

Infinite mana creatures you control can produce

Infinite untap of creatures you control

Stangg, Echo Warrior

Éomer, Marshal of Rohan

Helm of the Host

At the beginning of your combat phase, Rionya triggers, creating at least one token copy of Éomer with haste.
Put all but the newest Éomer into your graveyard due to the legend rule.
Declare Stangg and the additional legendary creature as attackers.
Stangg triggers, creating a 3/4 Stangg Twin creature token that is tapped and attacking.
Holding priority, kill the additional legendary creature.
When the creature dies, the Éomer token triggers, causing you to untap all creatures you control and to get at least one additional combat phase after this one.
Move to your next combat phase.
At the beginning of your combat phase, Rionya triggers, creating at least one token copy of Éomer with haste.
Put all but the newest Éomer into your graveyard due to the legend rule.
Declare Stangg as an attacker.
Stangg triggers, creating a 3/4 Stangg Twin creature token that is tapped and attacking.
Put either Stangg Twin token into your graveyard due to the legend rule.
Repeat from step 6.

Rionya, Fire Dancer

Declare The Incredible Hulk and Stangg as attackers.
Stangg triggers, creating a creature token.
When the creature token enters, Blasting Station triggers, untapping Blasting Station.
Holding priority, activate Blasting Station by tapping it and sacrificing a creature token, dealing 1 damage to The Incredible Hulk.
The Incredible Hulk triggers, putting a +1/+1 counter on it, then untapping it and causing you to get an additional combat phase after this one.
Repeat each combat phase.

Infinite creature sacrifice triggers

Bruce Banner // The Incredible Hulk

Blasting Station

Declare the creature with Breath of Fury attached and Stangg as attackers.
Stangg triggers, creating at least one creature token.
Deal combat damage using the creature that has Breath of Fury attached. 
Breath of Fury triggers, causing you to sacrifice the creature it is attached to, then attach Breath of Fury to the creature token, then causing you to untap all creatures you control and to get an additional combat phase after this one.
Repeat each combat phase.

Infinite combat damage

Breath of Fury

Samut, Hazoret's Champion

Combos with "Stangg, Echo Warrior"