This is one of the simpler achievements, it won't take that long and you will probably unlock it while playing.
if not;
I recommend using one of the longer game-modes (over 2 minutes long)
collect pellets and attract as many ghosts as possible, before long either one half or both halves of the maze will have a large number of ghosts on them, and you should have a lot of ghosts following behind you too, grab an orb and run through the ghosts behind you and then either of the large ghost clusters,
good job! you now have the 30 ghosts achievement! 
***I believe this achievement requires you do this without the orb mode running out, if you want a challenge
(because why not) you could eat the ghosts one at a time, eating the few orb-ghosts around the maze without the multiplier going down or the timer running out. it's actually quite fun.