http://www.tosbase.com/tools/skill-simulator/build/ow63t7vrck/
Full STR
New Class 
if Dragoon c3 Future it good
reset
SW3-HOP3-DRAGOON3
For 1H Spear and Shied Only–Skill Synchro Thrusting is high damage
There are no benefits on going cata since its more 2h spear. Just go Dragoon, really.
I gonna be honest with you:
Don’t choose a lower rank class in a high rank. You will be playing rank 8 content with a rank 6 class. On top of that, Cataphract is extremely weak right now due to overnerf, so i would recommend you to play with another build.
But if you really want to be a rider, i suggest Sword c3 -> Cata c3, double slash - rush.