Leif IV's

You got his strongest boon so I recommend you to build around that.

A Slot Darting Blow or his default Steady Blow will work perfectly, though aim for Swift Sparrow if you can. Fury is also a viable option.

B Slot you can leave alone if you highly value the -1 CD charge. Otherwise you can swap it to something more useful to a well rounded unit like Leif, such as a Chill skill or Renewal. If using Fury, Desperation and Vantage become great options as well.

C Slot is always flexible, but you can build the slot around triggering Light Brand's effect more often. Since Light Brand punishes having a gap between Defense and Resistance, you can run Threaten Resistance or Resistance Smoke This can also benefit you greatly if you're running Leif on a team with a mage, especially a green mage to cover Leif's weakness to blue.

Use whatever positional you'd like. As for the special, if using S Drink you can open yourself up to some higher CD specials like Ignis or Astra, otherwise Bonfire is always a safe bet, or Moonbow if you want something faster.

Seal is flexible, but you'll probably want to use it to patch up his attack stat with Atk +3. Close Defense works to boost his tankiness, or you could run dual Threatens with the Threaten Speed seal.