S27 Pakkun definitely. It seems Mental Power is getting more common now that people have figured out it has a great curve, great missions, and that Mind Transfer and Interrogation are really good in that deck. As it stands currently, Earth has no consistent answers to Pakkun though besides egging it out to battle somehow, or using something like Striking Shadow Snake or Moment of Weakness. All of those require considerable splashing.
I've seen people side Gameru in Lightning because it's on-element and sideboard-ish, but I'd say the usual negate like T2 Pakkun or Temari [GF] are better sideboard material. I'd much rather negate and discard than have it remain accessible. Plus Gameru's pool of cards it can negate is smaller. He's still an on-color possibility if it tickles your fancy though.
Some good sideboard staples are Shimon as you mentioned, Sabiru/A Quiet Day to halt nasty effects on block, Suigetsu [Dehydrated] to stop key effects like Naraka and Shimon on attack, and whatever else you find that can put a damper on commonly used strategies and counters to whatever might shut down your deck. Building a sideboard is all about being aware of the meta, the meta's weaknesses, and your deck's weaknesses.
I'd advise against Application of the First Stage because it's really doesn't provide your deck with answers to game-changing cards. Instead it's a situational jutsu that you'll only use against water decks that you can probably beat anyways.
Your sideboard might look something like this if you need Pakkun:
2x Sabiru/Quiet Day
Obviously subject to adjustments based on how important you feel they are. For example, if you really don't find Suigetsu all that useful you could take one out for a third Temari or another card your deck needs to keep functioning properly or to break your opponent's strategies.
Or you could get really creative with a sideboard that COMPLETELY changes the premise of your deck. Those are my favorites, but they leave you open to ridiculous things like perpetual Determination to Protect.
Rule of thumb: you can never go wrong building your sideboard with cards that protect your deck from unexpected effects or strike at the weak spots of meta decks. The composition of your sideboard should depend on how your deck interacts with the meta in general. From there it builds itself if you're willing to comb through the card search.
I spent a bit of time writing this, so yeah, pretty much what Earthbullet said, but elucidated a little.
EDIT: Forgot to mention Tailed Beast Sealing and Overwhelming Hunger as sideboard options. They're a bit more situational than the usual effect negate.
Last edited by NextoftheLegacy : 01-10-2013 at 04:14 PM.