Hello,
About what you say about the power of the team: The most powerful team is the one that is obtained from the Chiefs, this team can be improved, increasing the power of the team with gems. Also you can add stats.
This is a team game, since there are many instances that you can never do by yourself. That's one of the reasons why you should think about joining a guild (not diamonds) XD.
On how to get free diamonds: as Mazufir said and also killing everything you find in instances that you can do, batching what you throw the mods, selling it to an NPC and buying with that gold Diamonds in the central square of Varanas Cedric , in addition to many other ways. But you always need to recharge to be decent.
A greeting.
* I used a translator, I hope it is understood.