Concerning the relic (not shield use), Physical Mastery and Critical Rating relic would seem most useful for Guardian if aggro is to be based on physical damage skills. I think it's backwards, but prior to Tactical Damage Rating relic I was using a relic with +Partial Parry and Evade Mitigation. It's extremely minor but there isn't much else to do. I also wondered about the effects of a +Outgoing Healing and Critical Rating relic, if that affected any of our self-heals, but most are percentage-based. What we're looking at is a DPS class that wears a shield and uses taunts.
Maybe the next content update will have something.
Landroval: Tolvat (120 Guardian), Gelldir (115 Warden), Gloravon (115 Captain), Curwe (115 Champion), Glamdir (115 Rune-keeper)