Get yer 10 rockets and drag em to a hot key (mine was "T")
Basically set one off and then SUPER QUICK right after hit your hot key for it again and you can place another one instantly - Two casts in the time of one. Easy to set off 10 in 20 seconds using this method.
I know this isn't the most overly thorough explanation but it should be simple enough to understand.
I guess it was too simple an explanation for people...
you are casting 2 rockets in the time when you should only be able to cast 1....
I didn't think I would have to over explain it, but I guess there are some morons here who are either #1 mentally retarded #2 haven't done this achievement yet so they don't understand that normally you plant the rocket you then have a global cooldown on the rocket before you can cast it again
With my method you "replant" the rocket while the first one is still casting so therefore you bypass the global cooldown on the rocket and hence turn a 20 second achievement into a 10 second
JESUS DONT RESPOND AND FLAME ME IF YOU HAVEN'T TRIED IT - IT WORKS, YOU ARE BYPASSING THE GCD SO IT IS NOT INTENDED AND THEREFORE AN EXPLOIT