It takes ~ 12 rockets from the demolition class to blow a crate. Probably 50% of the crates can be hit from a comfortably safe distance. On some of the maps you can even hit the crates from the attackers spawn point.
Tanks can do some major crate damage as well, but it is kind hard to aim at them unless you are pretty close. The support class mortar strike can hurt it, specialist's C4 can hurt it badly. The assault class grenade launcher and grenades can do some major damage as well.
Just keep in mind that you don't have to set a charge to blow a crate.



