I’m a Highlander-Fencer myself. My build is Swordsman1-Highlander3-Peltasta1-Fencer1, currently.
I really like the sinergy between Highlander and Fencer. So you know, Ataque Coquille does the exact same thing Skull Swing does, but in an inferior way. Coquille makes pierce type attacks ignore the target’s defense for X duration, while Skull Swing drops the target’s defense to 0 altogether, but this applies to any kind of physical damage (Slash, Strike, Pierce & Projectile).
If your foe’s defense is 0 then there’s not really anything to ignore, is it?
That being said, Skull Swing’s downtime is higher than it’s duration, and lv 1 Coquille is nice if you want to keep ignoring your target’s defense with your physical attacks, but I’d skip it.
Here’s some mistakes I made while building my character, so try not to make them as well:
• I picked Pain Barrier at lv 1. Pick it at lv 5. You won’t regret it.
• I only have lv 4 cartar stroke. Put it somewhere between lv 7 and 10, at least. You’ll lack sufficient Strike Damage later, and this will fill your need just nicely.
• Vertical Slash is terrible. For god’s sake, someone remove it from my skill tree.
• Two-handed block can be better than you think, so long as you are not planning on have a high DEX build. You’ll regret it if you spend time trying to block to trigger the pierce debuff and your char keeps dodging hits. You’ll lose more DPS than you’ll get.
• The Ataque Coquille vs Skull Swing thing. Personally, I’d skip it (if I hadn’t already wasted a point on it). You’d think it’s damage is high and with it’s low CD it would help filling your Strike damage demand, but it’s only that good on paper. Since most of your skills are multi-hitting skills, you’ll soon learn that single-hitting skills are just plain bad.
One last note. As a Fencer, you’ll need to think ways to spend your SP efficiently. If you’re commited to heavy DPSing, even with pots your mana will drop REALLY quickly, so you’ll need to spend it only on skills that are worth it, SP/dmg-wise. So far, Crosscut+Sky liner and Moulinet are still worth it, and Cartar Stroke is too, but only if my target is vulnerable to Strike dmg. Sometimes it’s better not to cast anything and wait for the lunge debuff to cast Slash attacks since it’ll make most of your SP. Dievbirdys are your best friends, and they’ll allow you to spam as you wish 
Hope I helped your a little bit!