The priority order for EP skills for PvE Lancer is:
Main 3: Arush Constancy -> Regular Mighty Attack -> Onslaught Velocity
Next 3: Arush Force -> 1 point in Mighty Phoenix -> Arush Extra-Force
Get Constancy and Mighty Attack maxed first, then put points into Velocity until tier 5, then get Arush Force to tier 5 to balance the EP between the skills, then max Velocity all the way. Once Velocity is maxed out, focus on Force then Extra Force for Arush.
Once you've finished all the above you're pretty much done with the core EP for Lancer. Beyond this, you can go into Shield Counter Fatal Blow, Stand Fast Speed Skating: Retribution, Shield Bash Reset or Backstep Cooldown based purely on choice/ your playstyle. Or really anything else you want to try tbh. I choose shield counter stuff because it has a slight increase on dps sometimes. Anything helps at that point.