Always be Attacking - In addition to dealing bonus damage, Gwen's Attacks empower or reset many of her Abilities.
Gwen can still damage enemies outside her Hallowed Mists, especially with her Ultimate's range.
Some of Gwen's Abilities can apply her passive to multiple enemies, so aim them at groups to get maximum damage and healing.
Gwen's Hallowed Mist shroud will only follow her once, after that it will dissipate when she leaves.
Gwen must hit something to recast her Ultimate, try to evade her between casts.
Gwen needs to attack a few times to set up her damage, so try to get the jump on her.