Boon Chronomancer
A Giver's and Minstrel's Chronomancer — Rifle and Sword/Shield, carrying the squad's 
Stability on 
Mantra of Concentration.
That mantra is the only skill on this bar that gives allies Stability — the shatter trait people credit for it, 
Bountiful Disillusionment, stabilises you. What Chronomancer has that our other Mesmer support does not is 
Well of Precognition, a block window you drop on the pile, and 
Time Warp — which through 
Temporal Enchanter also hands 
Resistance and 
Superspeed to anyone standing in it.
Why these traits
Shatter 2 is the loaded button in Chaos: 
Illusionary Defense gives nearby allies 
Protection, 
Illusionary Membrane puts a chaos aura on you, and 
Chaotic Transference turns that aura into boons for the people around you. Bountiful Disillusionment stabilises you and adds one ally boon depending on which shatter you pressed — the squad's Stability still comes off Mantra of Concentration.
Seize the Moment is the 
Quickness: one application per clone shattered, plus more whenever a phantasm goes out. 
Illusionary Reversion hands a clone straight back on every shatter — with 
Flow of Time underneath it, that is where the 
Alacrity comes from too. 
Restorative Mantras and 
Mental Defense are ally-facing, not personal sustain: mantra charges heal the people around you, and shatter 4 breaks them out of stuns.
Gear reasoning
Giver's and Minstrel's with 
Water — healing power and boon duration, because almost everything this build gives out is a boon with a duration on it.
Concentration comes from 
Chaotic Persistence, which pays you concentration and expertise for as long as you have 
Regeneration on you. 
Rivers is what keeps it there: alacrity and regeneration at the end of every dodge roll. 
Energy refunds half your endurance on a weapon swap, so you get more dodges to spend.
Frenzy shaves cooldowns off your whole bar on a kill and 
Life stacks healing power on one — kill-reward fillers, worth the slots in a zerg and not much anywhere else.
When not to run it
If the squad is dying rather than being pinned, Troubadour is the better Mesmer. Its healing sits on the utility bar where you can aim it; this build's healing is a by-product — Restorative Mantras when you spend a mantra charge, 
Illusionary Inspiration when an illusion goes out.
Chronomancer earns the slot when someone has to hold Well of Precognition for the push and Time Warp for the follow-up.