It all depends on who teams up with who.

I think the warriors seen as the biggest threats will be targeted first, which may result in them either dying first or dealing the first devastating blows.

In my mind it goes down like this:

SANDOR and OBERYN, though unallied, attack GREGOR almost immediately...THOROS joins the fray with his buddy, ROBERT BARATHEON who have an alliance for the nonce. BARRISTAN follows ROBERT to protect him. Their combined power is enough to get the Mountain-that-Rides on his back for SANDOR to deliver the killing blow, however, THOROS' flaming sword pisses off SANDOR so much that SANDOR kills THOROS as well, in a flash of fearful fury. ROBERT, enraged at SANDOR for slaughtering his drinking buddy, attacks SANDOR, who is overwhelmed eventually by the skill of BARRISTAN at the side of ROBERT. OBERYN, after GREGOR's death, kills JOFFREY with a thrown spear, picking it up on his way to fight LORAS.

JAIME kills OBERYN while he is distracted with fighting LORAS to avenge the death of his son by OBERYN's spear. JAIME and LORAS then engage in a bloody and beautiful dance that leaves LORAS dead and JAIME blood drunk.

I expect JORAH would die at the hands of DAARIO in a brave but foolish rush to impress his beloved queen and kill the guy she's bangin'.

JON, knowing that Longclaw was meant for JORAH, his former LC's son, rushes to avenge the northman's death. EDDARD joins JON (obviously in order to protect him) and the two of them are enough to bring down DAARIO.

BRIENNE and JAIME kill VICTARION who is fucking up everyone's alliances by attacking everything he sees. TORMUND sees BRIENNE's impressive size and strength and tries to steal her from JAIME. He succeeds because both JAIME and BRIENNE are both exhausted from their fight with VICTARION