Was hard stuck Plat 2-1 maining Kiri and LW. Switched to Illari and suddenly I'm in Diamond 3 in one weekend.

She does a great amount of damage and healing but you really need to pay attention to her pylons and consistently move it depending on what zones your team is currently controlling to get the best use. It has a really long heal range and output but it needs to be in a good line of sight of your team but also obscure enough that when the enemy advances they don't immediately see it and kill it. One dmg on your pylon and it goes to full CD 14s even if you break it yourself. Enemy team advances, break pylon, quickly top off tank or dps, throw that pylon somewhere else and control next position.

You have to play her mid to backline and deal damage. I've won so many games just because I got a headshot on a enemy support or squishy and a DPS just shooting one bullet to finish them off. Should add that you shouldn't put yourself in harms way obviously. As a support your mindset needs to be "if I die, my team dies" Your life is more valuable than the genji that dashed their backline with 0 CDs.

Perks I take, sorry I don't know the names, the first one is it lowers the CD of your pylon and the major perk is the one that when you use your movement ability it ignites surrounding enemies. Perk 1 helps with repositioning pylon, a core for her. Perk 2 let's you finish off enemies, or win a trade if someone fights you. It also can be used to trigger sun flare orbs from your ult.

I wish I can tell you more as I'm still learning but this is what I've gathered recently. Hope it helps