Walking Dead wiki rulesheet Gives a great list of the multi kill starts, thanks to the maintainers of that thread!
List of game objectives that qualify Multi-Kill’s:
Three Barn shots during Barn mode.
Three CDC ramps during CDC mode.
Completing CDC mode.
Three Riot shots during Riot mode.
Two Tunnel shots during Tunnel mode.
Completing Tunnel mode.
Two Arena ramps during Arena mode.
Completing Arena mode.
Completing the Bicycle Girl hurryup.
Clearing the drop target bank 10 times.
Clearing the drop target bank three times during Blood Bath.
Scoring three jackpots during Well Walker multiball.
Scoring four jackpots during Prison multiball.
Completing three crossbow shots.
Collecting the Start Multi-Kill award.