Class Mastery
The Warden's Class Mastery Passives are incredibly useful:
Nature's Bounty activates on overhealing any one person with a Green Balance ability and provides everyone with major heroism for 4 secs.
Tundra's Maw debuffs enemies with major brittle for 2 secs on applying chilled to them, which is done with Winter's Revenge and Elemetal Susceptibility on the back bar to achieve 100% uptimes.
Sets
Pillager's Profit is on the back bar so warhorn will grant 60 ult to each person in the group. This has to be done with 500 ult but with major/minor heroism, a decisive trait and light/heavy attack ult regen, this shouldn't be too much of a problem. In an ideal ult drop, you are the last person to use their ult so everyone else gets theirs back faster. To keep track of the 45 sec cooldown, I recommend using Cooldown Tracker to set a 45 sec timer on the Pillager's effect.
Serpent's Disdain is on the body which extends your applied status effects by 16 seconds. This is mainly to extend the chilled effect which debuffs enemies with major and minor brittle. Essential for the group to reach the critical damage cap.
Ozezan procs on overhealing which is simple enough.
Skills
Aggressive Warhorn is the ult of choice to provide everyone with the extra recovery. This should always be dropped with 500 ult after everyone else has dropped their ults and within the group as the range is very short.
Winter's Revenge and Expansive Frost Cloak are slotted on different bars to keep up the increased chance of proccing the chilled status effect for Tundra's Maw. Elemental susceptibility will help with this as well.
Budding seeds should be placed within the DD stack whenever possible to trigger the Nature's Bounty passive for major heroism. Ideally, this should always be active for near 100% uptime on heroism.
The Illustrious Healing morph uses smart targeting rather than picking and sticking with the first 6 people in the aoe.