Strengths
If Obsidia is playing safe during the early to mid game, she can have a big advantage during the late game. Because if she is being babysitted properly by teammates, she can peacefully push, farm, or kill enemies.
Best Item for Obsidia
Very effective for Obsidia for that Critical Damage, survivability and range.
Emblem
For damage, I believe this emblem would suit her.
Battle Spell
Flicker is a nice spell for Obsidia, she can escape and chase after enemy heroes.
Advice
Always stack her Bone Shards or passive early on the game, try not go front line in clash to avoid feeding or giving enemies an advantage during clash.
Best With Carmilla
This could be a good combo, with Carmilla's zoning abilities and her ultimate, I think she'll be a fit fot Obsidia because of her continuous damage from her passive, they'll make a great impact during clash.
Watch Out For Belerick
Obsidia could be countered by Belerick because of his damage reflection, along with Blade Armour and Vengence.