Kari Zev does not have summoning sickness and cannot be blocked and killed by an opponent.
Steps
At the beginning of your combat phase,
Helm of the Host triggers, creating a nonlegendary token copy of
Éomer with haste.
Declare
Kari Zev as an attacker.
Kari Zev triggers, creating a legendary 2/1 Ragavan creature token that is tapped and attacking.
Holding priority, activate
Phyrexian Altar by sacrificing Ragavan, adding ({1} magic symbol) .
When Ragavan dies, each
Éomer that hasn't triggered this turn triggers, untapping all creatures you control and causing you to get at least one additional combat phase after this one.