You have to do the post-game fights, which means capturing Irroadium and Ziognioss. That means you also have to face all three stadium leaders in the post-game. It may or may not mean catching Tabalus as well...I'm not sure.
Grandlan does have the same issue, to some extent, but Grandlan's attack is higher than his special attack, and most of the moves he gets are physical, so he's one of those manamon you can alternately pump strength and speed into. And yes, he's weak to a lot, but at least most of his moves hit with a little authority. Only downside is that his strongest magic move is Aura Master (Meteor Rush is stronger as moves go, but comes off Grandlan's lower special attack). But with Grandlan, in particular, his stats are all at least decent, so if you want to for some reason split your train points even more, and want to put some in special attack so Meteor Rush hurts more, it won't suck.
Check out my Manamon text walkthrough at the following link:
https://www.dropbox.com/s/z8ls3rc3f4mkb … n.txt?dl=1