An Overview of the Ability Proficiencies for Lucas
Like the other characters, each of Lucas' abilities can be customised by investing points in one of two proficiences. A maximum of five points can be spent on each ability, with you being free to spend those five points on the ability's two proficiencies as you wish.
Shield Pummel: You can customise Shield Pummel to deal more damage or to gain a chance to knock enemies down. While you can go for the extra damage, which is the Brutal Pummel proficiency, I prefer the ability to knock enemies down personally. So, maybe split the points between the two while favoring Impetuous Slam.
Blade Dash: Blade Dash can be customised so that it deals extra damage to bleeding target or gain an increased chance to critically hit enemies. Personally, the extra chance to critically hit enemies is a good choice, so I'd put the majority of the points into that proficiency.
Graceful Repose: Graceful Repose can be altered so that Lucas gains health with each successful hit. The amount gained is dependant on the damage done. Also, it can be customised so that Lucas gains more power when he's hit. However, Live by the Sword seems more helpful, so I'd recommend pumping points into that proficiency so that Lucas heals quicker.
Vanguard's Strike: Vanguard's Strike can be customised so that it pushes nearby enemies away alongside slowing them or so it helps Lucas regain focus after defeating an enemy. Vanguard's Gale, the proficiency that pushes enemies away, seems more helpful here. Personally, I've never had any real problems gaining focus anyway. This is on both normal and hardcore difficulties.
Heroic Charge: This ability can be altereded so that it critically hits any enemy with less than 10% health, or so that it deals an extra 20% DPS (damage per second). Either of these choices are good really.
Shield Wall: This ability can be customised so that Lucas' block value is better. However, the Reflexive Defense ability is the best bet honestly which makes the ability reflect the enemy's damage back at them. The reason for this is because evading enemy's blows is far more effective than blocking.
Wind Shear: This ability can be improved so that it slows enemies down or makes them bleed. I'd put more points into slowing enemies down here (but still put 1 or 2 into Wounding Gale). However, if you focused on upgrading Blade Dash so that it dealt more damage to bleeding enemies, then do feel free to invest more points into Wounding Gale.
Earthrending Strike: The ability can be customised so that it can knock enemies down. The ability's range can also be extended, making it useful when enemies are crowding you.
Unbridled Wrath: The ability can be upgraded so that Lucas avoids interruption when it's activated or so that his focus regeneration is increased. I'd recommend investing in the focus regeneration proficiency myself, though I don't find that particulary useful.