This one is tricky as you have to obtain a x100 combo in Ghost Combo mode. You'll have to re-learn the routes and patterns that you are accustomed too to master Ghost Combo. The basic beginning strategy, is ignoring your train as much as possible. You may be tempted when you get that first power pellet to eat your whole train, but you want to save the ones with power pellets inside of them later on in your combo.
You'll want to seek out the stationary ghosts with power pellets inside of them to sustain your combo, nabbing the ones in your train or the center power pellet only if your about to lose your combo. You'll have to calculate your risks as to which ghosts to go after and which ones to skip. Here is a great example of the strategy you should employ.
If you choose to do this on Score Attack instead, you have to get this all in one 5m session, but not in a combo. When you're going for a high score you'll want to eat as many ghosts as possible anyways for highest possible points. Ignore the power pellet in the middle unless necessary as it wastes a few seconds getting to it. I found this to be more difficult to accomplish rather than in Ghost Combo.