The Rose of Remembrance
You'll definitely want Triss to lead the way for this one. Just follow her out through the city and into the forest. Keep an eye out for any monsters, although they should be easily dispatched by the two of you.
The end goal is the waterfall above the cave that had Ostmurk in the Kayran quest. Just follow her up and listen to the conversation, then move into the hidden garden. You can then pick the Rose of Remembrance from the bed of flowers in front of the statue.
Once the next conversation ends though, you'll see the group of bandits that were apparently sent to steal the statute. This doesn't sit well with Triss, so get ready for a fight.
The hardest part is the fact that the three bandits will start out right next to you. On the bright side, Triss is here to help with a few fire spells and her dagger. Start by dodging away from them. If you can get a little distance between you, you should be able to hold the Axii spell long enough to actually convert one of the bandits to your side. This makes the whole fight much easier.
While the converted one fights, you can just parry the blows of the odd man out, and take him out with a simple riposte or counterattacks. Aard is also good for opening them up for big hits. Once the Axii wears off, take out the weakest bandit first, then pair up with the last one. Triss will try to help with some spells, so keep a little distance between you and the remaining fighter. This will prevent friendly fire. After that, just parry and then counterattack, and you'll be fine.
After the cutscene you end up in the ruins of the underground bathhouse. I suggest that you look in the chest in the corner. You can get some red meteorite ore and a quest item, a book about The Wild Hunt. If you wish, you can trigger a sex scene with Triss by suggesting to her that you use the bath in here. It's up to you. If you don't want to do it, you can use the Aard sign to blow open the wall by the exit.
If you choose to have a bit of fun, another cutscene will play. After a conversation with Triss, where you'll make some big story choices, Roche will knock open the wall for you. You can talk to him too, if you want to update him on what's happened.
That's it for now though. Make your exit and continue with the quest.
Note that you can talk to Dandelion about the manuscript on the Wild Hunt that you found, if you wish.