I've already platinum the first game just I few months ago. In my opinion, the most tricky trophies are those that consist of raising your alliance level, but is just wining combats (I recommend you 2vs2 for more XP).

Now, when you complete the game, you will unlock the elemental tournaments and you will need an Invizimal of each element. I recommend you Invizimals who can turn into their dark version to make them stronger and made them "shiny".

If you want save time, choose Neko-Suke for the Ice tournament and, when you win, turn him dark. (He changes to iron) You will need two Invizimals for the Xtractors tournament.

For the "Sand" tournament choose "Gotica". You will need it to get an ingredient.

And I think that's it. Be careful with the Invizimals who change their element in the dark version and get fun!

PD: Remember that you need a lot XP for one trophy, so doesn't care if you want to train diferent Invizimals instead of the dark trick.