Molten Echoes | Cordial Vampire | Phyrexian Altar (and one other card)

Cards

Cast
Dusk Legion Sergeant by paying ({1} magic symbol) ({B} magic symbol) .
When
Dusk Legion Sergeant enters,
Molten Echoes triggers, creating a token copy of
Dusk Legion Sergeant with haste.
Activate the token
Dusk Legion Sergeant by sacrificing it.
When the
Dusk Legion Sergeant token dies,
Cordial Vampire triggers, putting a +1/+1 counter on each Vampire you control.
Resolve the token
Dusk Legion Sergeant's ability, giving nontoken Vampires you control persist until end of turn.
Activate
Phyrexian Altar by sacrificing another nontoken Vampire with base toughness 2 or higher, adding ({1} magic symbol) .
When the Vampire dies, its persist ability triggers, returning it from your graveyard to the battlefield with a -1/-1 counter on it.
When the Vampire enters,
Molten Echoes triggers, creating a token copy of it with haste.
Activate
Phyrexian Altar by sacrificing
Dusk Legion Sergeant, adding ({B} magic symbol) .
When
Dusk Legion Sergeant dies, its persist ability triggers, returning it from your graveyard to the battlefield with a -1/-1 counter on it.
Repeat from step 2.

You may repeat steps 6 through 8 as many times as desired to get infinite token copies of nontoken Vampires you control with base toughness 2 or greater. You can omit the additional Vampire if you control a static effect that boosts your creatures' toughness by 1 or more.

This combo has been estimated to be Exhibition (Bracket 1+).