Focus is very difficult to get by eye in such a situation.

Zoom in a snake head. Follow the head, keeping it centered.

Now switch ON AF until focus is achieved. Now switch it OFF.

(If you can't get focus this way, it's possible you are simply too close.)

You can double check using Expanded Focus.

Now you can zoom back a bit to frame the scene better. Zooming back increaeses your depth-of-field which is good.