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Explorer
I'd also recommend a few swaparounds. Take Form of Mountain's Thunder as your L5 Daily - it's a solid offensive AND defensive boost, and lets you further benefit from items and feats that give you bonuses while in a guardian form.
I'd also encourage grabbing Weapon Expertise instead of Markings of the Elements. That's a very situational feat, requiring opponents to be dealing elemental damage AND hitting you before it's going to be useful.
Vicious Advantage is nice, but also situational. Good ol' Toughness will give you a bit more of a defensive edge, and can optionally be retrained at Paragon for Enhanced Font of Life for a +1 to saving throws.
I'd also encourage grabbing Weapon Expertise instead of Markings of the Elements. That's a very situational feat, requiring opponents to be dealing elemental damage AND hitting you before it's going to be useful.
Vicious Advantage is nice, but also situational. Good ol' Toughness will give you a bit more of a defensive edge, and can optionally be retrained at Paragon for Enhanced Font of Life for a +1 to saving throws.