I suggest investing in this tree as the lowest priority. Due to this, I do not recommend them as they need additional micromanaging. However, they do not use their skills by themselves. You would then have an additional companion in the battle: Wolf, Bear, etc. You can also go for Ranger specialization. This will benefit almost all of Leliana's attacks and talents. We choose it because it provides +2 dexterity and increased critical chance. This is beneficial for us as we can start using songs early in the game to get their benefits for the whole party. Leliana comes with Bard specialization by default. This will give some additional nature resistance. What is left you can put into Survival.After that invest to 3-4 levels into Tactics so that AI could comfortably use all the needed talents.Master Combat training, this row will provide: bonuses to attack, armor, and higher tier weapon talents.
You can also invest in the constitution for more survivability, but I view it more as a waste of points for this type of character as it should be avoiding direct combat. Moreover, after you get Lethality talent it will boost your damage directly. Cunning will boost your songs increasing the bonuses that they provide.
I suggest this investment into attributes: Moreover, to wear some of the best armors at the end game you will need strength. As Leliana is a rogue, your main attributes are dexterity and cunning.