Hey, this is looking cool. I like your color choices and your enemy designs. I think the biggest problem with the perspective is that the large slime and the tree creature are drawn from a front view whereas the rest seem to be in more of a 3 quarter view. Here's a quick attempt to bring the perspective in your tree creature in line.
I don't know how well I managed but I'll try to explain what I did.
- Foreshortened the trunk like you did in your newer trees
- Increased the area of the top of the crown and reduced the shadow
- Tried to move the legs below the base of the trunk
- Tried to add some depth to the mouth and the eyes
I don't think I was completely successful, but maybe it'll give you some ideas!