God Serena has spells that will do damage to Motherglare, but I'd say the dragon can win this with a oneshot.
Dragons can fly, and God Serena has not shown the AoE to take down a flying dragon. Conversely, Motherglare can fire a roar that can level a mountain. God Serena's durability is not high enough to take on Motherglare's roar. The difference in their bodies to take on damage is huge. The physical strength of a dragon is immense. A flick of the tail could send Serena flying. A punch might KO him, and a bite would kill him. So God Serena has to avoid physical confrontations, and he can do this by using his spells.
So in my scenario a flying Motherglare can oneshot God Serena with a roar. If God Serena roars at the same time, then maybe he can stop it. But I'm not sure how big his roar is, and I'm only giving him the level of spells he used against the Wizard Saints.