Ynnari require different paradigms.
The idea is instead of finishing off units, you want to weaken them as much as possible at range without killing anything. This is counter to the logic of every other opponent and it makes decision making very difficult, especially when combined with things like independent characters (like solitaire) or the jumping Ynnari avatar.
Another thing to keep in mind is the 7'' range. When you kill a unit, kill the one bridging the gap between other units, cutting the soul burst "chain". Also, keep your units (especially squishy drone squads) at least 7''+move range away from enemies to stop their use as a catalyst.
Also, Ynnari work best bunched up, but most eldar good stuff is close range. Thus, spread your army to pull them around the board.
Also, no matter what the eldar player says, each unit can only soul burst once per turn, keep them honest about it.
Augmented Puretide Council
The Tau Deathstar 2015: 8-1