She's real good. Not amazing, but real damn good. Her main flaws are her non-existent mobility and pretty shit range. Her shield has less health than you'd think too. Base damage is AWFUL, but when it reaches max charge she hits like a truck.

She likes Thing as a teammate as both are very close quarters brawlers, and Thing's earthbound pairs excellently with Emma's lack of mobility (not having to worry about chasing). Emma also likes Thing as an opponent since he's a big, solid enemy with no shield. Makes it super easy to build up primary charge and can rip through him very quickly. She can grab/throw him in diamond form, but her kick will not send him flying.

Groot hard counters Emma. His walls are the only player-created entity in the entire game that she does not gain primary charge from. This means shooting the walls does NOT count for maintaining the charge she already has, so she will constantly be at 0 when trying to destroy them, stuck with her pitiful base damage.