Wish on the appropriate banner - Rosaria is a 4-star Cryo polearm user that can be obtained through the game's gacha system. You can try your luck and make wishes on the limited-time event banners that feature her.

Participate in events - Sometimes, the game holds events that allow you to obtain Rosaria as a reward for completing certain quests or challenges. Keep an eye out for these events and participate in them to increase your chances of obtaining her.

Buy her from the shop - Occasionally, Rosaria may appear in the game's shop for players to purchase using in-game currency or real money.

Wait for a future banner rerun - If you're not in a rush to obtain Rosaria, you can wait for her to appear again in a future banner rerun. This may take some time, but it's a guaranteed way of obtaining her without relying on luck.

Keep in mind that obtaining characters in Genshin Impact is heavily reliant on luck, so it's important to manage your expectations and not get too disappointed if you don't obtain Rosaria right away. Good luck!