If Dockside Extortionist is in your hand, cast it by paying {1}{R}.
Otherwise, cast it from your graveyard for its escape cost by paying {1}{R} and exiling any three cards from your graveyard except Brain Freeze.
Dockside Extortionist enters the battlefield, triggering itself and Death Match.
Resolve the Dockside Extortionist trigger, creating at least five Treasure tokens.
Resolve the Death Match trigger, giving Dockside Extortionist -3/-3 until end of turn.
Dockside Extortionist dies as a state-based action due to having zero or less toughness.
Activate four Treasure tokens by tapping and sacrificing them, adding {2}{U}{R}.
If Brain Freeze is in your hand, cast it by paying {1}{U}.
Otherwise, cast it from your graveyard for its escape cost by paying {1}{R} and exiling any three cards from your graveyard except Dockside Extortionist.
Brain Freeze's storm ability triggers, creating a copy of Brain Freeze for each other spell you've cast this turn.
Resolve the first two Brain Freezes, causing you to mill six cards.
Resolve all remaining Brain Freezes, causing target player to mill three cards for each Brain Freeze.
Repeat.

Infinite self-mill

Near-infinite colored mana

Near-infinite death triggers

Near-infinite ETB

Near-infinite LTB

Near-infinite mill

Near-infinite storm count

Near-infinite Treasure tokens

Death Match

Dockside Extortionist

Brain Freeze

Underworld Breach

Combos with "Death Match"