"Watch out for that tree!" said Newt, Jacob, and even Grimmson in unison.
Fortunately, Tina missed the tree and landed elegantly on the ground with her wand aimed at Grimmson. However, Nagini smacked into the tree, but no one noticed because she's a pointless character.
"It's over, Grimmson!" declared Tina. "We have you outnumbered three to one!" Of course, it was actually four to one, but... well...
"There may indeed be three of you," said Grimmson, not noticing Nagini either, "but only one of you has a wand, and the one who does is just a girl!"
"Just a girl?" Tina repeated. "Don't get a big head, Grimmson. Engorgio Skullus!"
Grimmson's head swelled up. "Melofors!" he replied.
"NOOOOOOOO!" cried Newt, diving in front of the spell. It hit him, and his head turned into a pumpkin!
You might think this would be a great time for Nagini to use her snake power, but frankly, even we've forgotten she's in the story at this point.
"Anteoculatia!" said Grimmson. Newt tried to dive in front of that spell as well, but he couldn't see with the pumpkin over his head. The spell hit Tina, and she grew antlers. Grimmson laughed evilly as he sauntered up to her.
"Diminuendo!" said Tina. The spell shrunk Grimmson small enough for Tina to crush him under her foot. He had no choice but to turn and run.
"You may have won this round, woman," said Grimmson, his voice now hilariously high-pitched, "but I'll be back!" And with that, he Disapparated.
By the way, it turns out that even Grimmson wasn't evil enough to not care about climate change. This is canon now.