You control another legendary creature that can attack this turn.
You have a way to kill the additional legendary creature at instant speed.
Steps
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.