Good Jubeat Plus songs to practice technique on?

I can clear almost any level 10, mostly with an A or S grade but I can only manage to get a B on the more difficult stuff like IX, 負けないよ･･･-Runners High-, ススススペースハリネズミ, Niflheimr and Evans to name a few with pretty bad timing as well and I'm unsure whether it's just the age of my iPad hindering me or the fact I'm not skilled enough to get a higher grade on these songs.

Are there any good songs I can play to practice some techniques used on these songs as well as others?
Also regarding notes which appear one after each other in a line stream, how is the best way to execute these?

As a level 9 player, I don't think I can help you with good level 10s to work on, but I have a few techniques for line streams. For horizontal line streams, I line my fingers over all 4 squares and kinda roll them across, as if I'm playing Stepmania. For vertical line streams, I just drag my finger downwards. (I've never seen an upward stream...) It's actually surprisingly accurate, and it's worked for me on the arcade version too.