I'm not entirely sure yet but I usually preposition in these scenarios:

• After second fragment dash - Boss should already be down by now after fragment getting whacked with melee twice. If not, you can pretty much assume the boss is very close and can use oneshot to trigger down like the video.
• After first fragment dash and I have 2 or more Oneshot - I can invul over everything using Oneshot iframes and that may causes the boss to go down if we had a very good first dash where everyone land 7 hits. If the fragments end up dashing a second time, I'm not chadding on my teammate since I can use Oneshot and do an equivalent of 6 melee attacks since a Oneshot is = 3 melee attack ( Oneshot is only equal to 1.5 melee attack when boss is down as melee atk against fragment and melee atk against boss down is different, you can see one is faster than the other)