This much should give you enough synergy to trigger your commanders abilities while leaving room for ramp, removal, and lands
Yes, Mega Gengar EX is one of the top chase cards and is worth pulling in Pokemon TCG Pocket
detach all materials from it, and if you do, return it to the Extra Deck, then, if there is a Monster Card in the GY among those detached materials, Special Summon as many of those monsters as possible from the GY to your opponent's field in face-up Defense Position
There's a memory component here, but it's no harder to track than, say, the Day/Night cycle while there are no Daybound or Nightbound creatures in play
Again, the key is being able to put counters back onto the Willow to turn it into a proper graveyard value engine