Simple. Easy as pie. Consider it done.

Freya had been paranoid enough to practice her patronus charm several times last term in the Room of Requirements, so she already knew what to do pretty much.

She raised her cherry wand and began to think of what made her happiest. There it was again, that long forest path near her old home, but she could hear someone nearby her laughing. The memory was so suddenly vivid that it frightened Freya for a moment. Hollis was standing next to her and walking along the same path, and like he usually did when he was alive, he was laughing, (probably at some joke he'd just made about being feral). Freya could almost feel herself shrugging the boy's antics off when she remembered what was supposed to be going on-

"Expecto Patronum!"

It almost sounded like a muffled cannon blast, and a very large silver shape serpentined through the air for a moment before becoming it's true form. Freya's massive saltwater crocodile swam through the air and looped around it's master a few times before snapping at something invisible in the air and instantly going into a death roll to break whatever it had seized apart.
Freya smiled as she looked at it, there wasn't even a hesitation for it to become a crocodile today, which made her very happy.
Though, looking at her patronus again, it did look somewhat larger than usual. The body seemed more muscular and the tail even appeared stronger. 'Probably because the last time I conjured it I was slightly ill. I guess now it's in it's peak.' But that was weird, shouldn't it always...eh, she'd wait and test it out some other time to see if the results were the same.

Freya's crocodile eventually floated over to it's creator and encircled the girl; it's mouth wide open and looking quite menacing. It didn't bother Freya at all though; however she could almost hear the animal hiss. 'It can stay as long as it wants...'. The girl had a small smile on her face as she once again was lost in her pleasant daydream.