I have a overall 64% winrate with her and an 88.89% winrate in the last 3 months @ 3.5k MMR, so maybe I can help a bit.

First thing is, don't play her as an initiator. Mainly use her ult to delete a support so that the fight becomes a 4v5 (as you already do). If you see the opportunity, just use it because, if it's a support, the support will dissipate in like half a second. The invulnerability won't be an issue against your team.

Second, play her as a defensive support. Get glimmer cape and force staff to protect your allies, get euls and ghost scepter to protect yourself, and get arcanes to help with mana issues.

Third, you can use her ult to quarantine their carry for a few seconds. If your team initiates a fight and you see their carry coming in, ult him so he can't join the fight for 3.5-5 seconds. That's PLENTY of time for your team to either get away or clean up the other 4 enemy heroes.