The "Arbalistic" advancement requires the player to kill a giant and an illusioner as one of the 5 "unique mobs" even though they cannot spawn naturally. It is impossible to do find these mobs in survival mode because giants simultaneously require a light level greater than 11 and less than 8 in order for them to spawn, while the illusioner can only spawn from the /summon command.

How to reproduce

1.  /summon illusioner and /summon giant 

2. Spawn 3 random mobs

3.  /effect give @e poison 999999 255 

4. Get a Piercing IV (4) crossbow