Kassian finished his own rebound to get the Amerks on the board at 1:43 of the second period, halving the Marlies lead to 2-1.
After the Marlies regained a two-goal lead in the third, Corey Tropp started a passing play that resulted in Legault's fourth of the year.
Trailing 4-2 in the game's final minute, Kassian tipped in his second of the game and 13th of the year with just :04 to go to complete the scoring.
The Amerks (21-18-6-3) remain in second place in the North Division.
Mike Ryan earned an assist in his first game back since October 14th.
The Rundown:
Kassian: 2G, 4 SOG, +1
McNabb: 2A, 3 SOG, +3
Tropp: A, 1 SOG
Schiestel: +2
Fienhage: +1
Lagace: even
Crawford: even
Persson: -2