I have 600k power and i am N1 at Cerber and N2 at Baal, also last hitting. Sometimes there is still situations, when some weaker players last hitting, but lets get things clear: my dmg vs boss is 200000, damage of second guy is 110000 and third is 90000.
When Boss have about 190000 HP left, i will attempt to last hit and will strike-kill him in one shot, but there is still a chance that those number 2 and 3 will attack together at once and they will suddenly last hit him before i do, with chance like 50%, but for other players with dmg like 50k - its almost imposible to last-hit-defeat him and their chance is under 5%.

200k dmg - my kill chance is 50%.
110k dmg -chance is twice less - 27%.
Third guy 90k - 23% chance.
45k dmg is 12% chance.
22,5k dmg 6% chance.

etc... its not 100% accurate numbers, but they will give you a clue about possible chances to win. In reality number 2 and 3 players, will newer kick boss in the same second, cus boss attack cooldown is 60 seconds and its 1 to 60 chance to fight him in the same second, cus it requires about 35 attacks to finish boss, and we cant just fight 100% accurate, cus boss is moving around and the road to him different every time... ^^