You can use the mana generated with Rosheen's last ability in any cost you pay that includes {X}, including the mana costs of spells, activation costs, and even costs for special actions (such as morph). Any time you pay mana, that's a cost.

You don't have to spend all of the mana generated with Rosheen's last ability on the same cost.

A "cost that contains {X}" may be a spell's total cost, an activated ability's cost, a suspend cost, or a cost you're asked to pay as part of the resolution of a spell or ability (such as Condescend). A spell's total cost includes either its mana cost (printed in the upper right corner) or its alternative cost (such as flashback), as well as any additional costs (such as kicker). If it's something you can spend mana on, it's a cost. If that cost includes the {X} symbol in it, you can spend mana generated by Rosheen on that cost.

You can spend mana generated with Rosheen's last ability on any part of a cost that contains {X}. You're not limited to spending it only on the {X} part.