Check out this video where a Hammer Fiona fights the Blood Lord solo on Hard mode.
This boss is in the Shadowed by Darkness battle quest. The player is Aciro from the Vindictus European server. You can see the forum post here.
Pay attention to what she does when she blocks. When she perfect guards (green spark on the shield when hit) she transitions into Butterfly Swing with two moves: left click and right click. That’s an indication that she has at least Guard Rank 9, which means that when she perfect guards, she gets “given” two free combo points. Normally a Butterfly needs 3 normal attacks before the smash rather than 1. This is a player who has studied the timings, doesn’t get over defensive and tries to do the combos that deal the most damage in between the Blood Lord’s attack.
This is the trick with Hammer Fiona. It’s knowing when you have the time to perform certain combos in between enemies’ attacks.