Choose Sacrifice over Magic Crash. Sacrifice 1 is like the old max Roar in terms of HP burn even if it does do the same damage every level.
Magic Crash doesn't work on Weapon/Magic Cancels, no (outside Monster Carnival and Chimera, there are regular monsters that can Cancel?). It works on other buffs a boss can cast (Damage Reflect? Maybe not =P). I don't know if it prevents Cancels from being cast, but it only lasts 20 seconds at max.
If he's 150 or higher, then he should just skip Dragon's Blood and pump Hex. If he does 3 Dragon's Blood (Magic Crash's req), 11 Iron Body, and 19 Mastery, he can max every other 1st-3rd job skill.
Magic Crash doesn't work on Weapon/Magic Cancels, no (outside Monster Carnival and Chimera, there are regular monsters that can Cancel?). It works on other buffs a boss can cast (Damage Reflect? Maybe not =P). I don't know if it prevents Cancels from being cast, but it only lasts 20 seconds at max.