The way Tonks saw it, she had two things she had to do. She needed to protect Marcus, and she also needed to nullify the lizards. The problem was, anything big enough to hurt the group of lizards would also be big enough to hurt her lover. That just wasn't going to work for her.
She barely thought. She was an Auror, and trained in a variety of spells that most wizards and witches would never use. It was time to bring them out. So she flicked her wand, and her Patronus in the shape of a large lizard came out. "Protect Marcus!" She shouted to it, and the thing floated over and covered the Ranger in a haze of silvery light.
Of course, the spell had been meant to use against Dementors, not lizards. It probably wouldn't hold for long, and time was running short. Tonks squeezed shut her eyes for a moment, but opened them with a new look of determination on her face. Her voice was calm, despite the fear she felt.
"Imperio." She spoke the word of the Unforgivable curse, and pointed her wand at one of the lizards. She needed it's help. "Fight that one." She commanded the creature that she had complete mental control over, and it obeyed. The two of them ran off, fighting. Which left her with only two more. Deep down, she was shaken by having used one of the Unforgivables, but she knew she would willingly do such a thing again if it led to Marcus being alright.