Ankabut had been as overjoyed to see the oasis as anyone else in the group. She had beaten Akilah to the side of the water and was busily filling skins and gourds. There was laughter. The water itself seemed to dance.
Nothing had fallen into it.
Her arm came out, catching Akilah across the chest and knocking her back a step so her handmaiden could step between her and the water's edge.
Bursting from below the surface came not one but a multitude of twisting serpents, hissing and writhing! But no...one?! It was enormous! And with so many heads, how would she control who it attacked? The answer was clear as soon as the question occurred to her though. How did a spider attack a man?
Palm trees were dotted around the sides of the pool. One was close enough, she thought. Her deep brown eyes locked on Akilah's, and she said,
"Stay back." Then she ran at the tree. Her feet touched the rough, scaly bark of it and she twisted her toes just so...and suddenly she was not running up a tree trunk. She was running along it, and the world stretched up above her like a vast wall. The water in the pool should have come sloshing out, spilling over her and sending the beast falling endlessly down...down to the opposite horizon far below.
Amina got far enough out on the tree trunk, glanced back up over her shoulder...and leapt backwards, tucking into a flip as she went. The moment her feet left the palm, the world whirled around her. Down was the ground again, and she was hurtling towards the monster over the water.
Hoping to land squarely on its back!
(Using the 'Climbing a larger creature rules on p.271 of the DMG. Making a Dex (Acrobatics) check as an action, which it can oppose with its own Dex (Acro). If I win, I occupy its space and move with it, and have advantage to attack it. Its ability to attack me is subject to GM's assessment, though it's necks are long and snakey enough that I expect it probably can attack me freely.

It can also make a Str (Athletics) check contested by my Athletics or Acrobatics, to dislodge me.)
Dexterity (Acrobatics)! [roll0]
(Note- If you rule the distance to the hydra is great enough that I'd need a die roll to reach it, I would like to spend a Ki point as a bonus action to gain Step of the Wind, which doubles jump distances among other things. If I do -not- need Step of the Wind, I will instead use a Ki point as a bonus action to gain Patient Defense, which allows me to Dodge; disad on all incoming attacks this turn until my next turn begins)