The player who's turn it is (Necrons in this case) determines the order for things that happen at the same time, like
MSS and challenges. So he would probably challenge then trigger. Big reason not to let Necrons charge you.
The
FAQ also address the way whips work.
Page 44 – Canoptek Wraiths, Whip Coils.
Change the second paragraph to read “Any enemy model that is in base contact with a Canoptek Wraith with whip coils at the beginning of the Fight sub-phase counts their Initiative value as 1 until the end of the Assault phase, regardless of their actual Initiative.”
So that is up to the controlling player as well.
Hypnotic gaze is in the
FAQ, it says that it can be used in the controlling players turn and after challenges are declared.
Page 62 – Hypnotic Gaze.
Change the first sentence of the second paragraph to read “This psychic power is used at the beginning of your Fight subphase, after any challenges have been issued and accepted or declined”.
So to summarize. Necron can choose to
MSS or Challenge or have any models in
BtB with a whip go to I1. They decide the order. Again, not good for Necrons to charge you.