I tried earning stars on my Platinum but it really is difficult. So far I've only got one star for beating the Elite Four. Since I'm not a fan of the Sinnoh Pokemon contests I'm not going to attempt the Master Contests anytime soon. Right now, I'm slowly working on getting a Platinum flag underground and a 100 trainer battle streak in the Battle Tower along with catching them all.
Heartgold is also one star,I'm planning to win all the Pokeathlon courses since I find them really enjoyable :3. Also I've heard the Shiny Leaves are easy to get if you know where to find them for a certain nature so I'm certainly trying that soon
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