The BA book powers have some of the best that were created in 5th edition. Shield of Sanguinius is no longer as good with the reduced requirements for cover. Blood Lance is really a glorified melta shot which is not as good as it was with vehicle rules changes. The Sanguine Sword and Unleash Rage also feel underwhelming with the decrease in value of assault in 6th edition.
6th edition is more of a shooting game now. Prescience has been well recognized as one of the best powers in the rule book due to its reliability and effectiveness:
Primaris Power
Prescience is a blessing that targets a single friendly unit within 12". Whilst the power is in effect, the target unit can re-roll all failed To Hit rolls.
The important thing is to look back at Blessings:
Blessings are manifested at the start of the Psyker's Movement phase. They grant extra abilities to the Psyker's allies, such as characteristic boosts or additional special rules. Blessings can affect units that are locked in close combat and can affect the Psyker himself. ...
How can this power do the most work for your army? It's probably best to not upgrade your Librarian to an Epistolary as that is half the cost for an additional Librarian. The To Hit roll is generally the biggest limiting factor to damage output (the first step and only 66% chance).
What you are going to do is have your Librarian in Rhino / Razorback deployed within 12" of your Devastators. You want to cast it Turn One on them to maximize your chances of putting damage onto your opponent.
You have some flexibility about your approach for what comes next. You can use Sternguard or Death Company (without Chaplain) and go with five and a razorback with Heavy Bolter or Heavy Flamer or a larger squad and a Rhino. Then you can zoom ahead still on Turn One. Do not leave them unsupported or they will die as usual.
You will then apply battlefield tactics like you can see on YTTH to maximize your firing and cover potential. Those re-roll hits will be maximized with those two units as you will pretty much always be shooting with them both and probably end up in hand-to-hand with two attacks. It's best to not waste points on Sternguard power weapons, just have them shoot and move back which will be hard because you want them rapid firing. Remember, you will get re-roll Overwatch Rapid Fire Snap Shots too.
Last, do not forget about your Rhino / Razorback after disembarking. It still can fill useful battlefield roles. Additionally, do not forget that you can give the Prescience blessing to a unit that you are not in, and that unit could already be locked in combat to give them some help.
This could work just as well if you are running a bike squad, just put your Librarian on a bike too. You could also do this with Sanguinary Guard, especially if they are troops too.
You can always double this up with a second Librarian to further harass your opponent.
Using Prescience for your army might be a refreshing way to play your Blood Angels army.
