The trait Tireless Defender in the Defender of the Free tree adds a buff to this skill which lowers the power cost of your skills by 30%.
The trait Guardian's Ward: Tactics in the Defender of the Free tree adds a Tactical Mitigation buff to this skill.
Can have a considerable effect on the outcome of tougher battles with multiple mobs and/or those of Elite difficulty or higher, so it is a good idea to keep it up at all times in these situations, and most others as well.
Stats Table (sampling various levels)
|Character Level||Parry Rating||Block Rating||Power Cost|