Unlike some of the previous editions of Fantasy, the current edition allows for simultaneous attacks from both sides. If both units have Always Strike First (ASF), Always Strike Last (ASL, most frequently from great weapons), or the same initiative then you just resolve the attacks at the same time.
Yes, that means that an I9 Elf Assassin with ASF strikes at the exact same time as a Skeleton with Vanhel's Dance cast on him
