Ok, the full situation was a Landing craft had arrived on the edge of the table by planetfall containing Devastators and Tacticals, both in Land Raiders. Both formations elected to stay in their vehicles to provide a better range for their initial movement, and the Tacticals had moved away. The Wytches arrived in a Vampire, getting a hit from the Landing Craft AA and then had to decide whether to count the Landing Craft as intermingled or not.
The Wytches CC3+ / FF6+ first strike caused 7x hits on the Land Raiders killing both of them but 3x Devastators saved setting up the situation. Both sides made better than average dice up to this point (should be 3.3 First Stike hits potentially not killing any LR or Devs). I was counting on the LR saving, the Landing Craft killing 2-3 and with the Eldar BMs this should have left the Devs 3+ minimum - but statistics do not count where I am concerned
To the question about First strike being a liability, Neal is right that it is not a liability because the extra kills should more than balance the additional attacks of the surviving troops. Consider the potential events if this had been set up against Tacticals in Rhinos:-
With 6x Wytches on CC3+ and 2x FF6+, against a measly 3x CC6+ it is no contest on the face of things. The Wytches should score ~4.3 hits, killing all three Rhinos and also three further marines. In reply, the Marines get only 1.5 hits, and even if that kills two Wytches, the Wytches go into the assault resolution +4 up(kills and outnumbering), so it is highly unlikely they will lose. If the Marines save two Rhinos, they should only get ~1 hit in reply, and the Wytches go into the resolution +1 up. The marines only enter the resolution ahead of their opponents if they manage to save all three Rhinos (which is highly unlikely - unless you are using the fabled "left-handed Dice of Dave" when it almost a racing certainty)