The only one I don't know about is the boar butt. If I put space in between him and the tree wont the space be noticeable and create the same kind of effect? I guess it's better than it looking like he's sitting on it.
The best thing to do would be to alter either the pig or the tree so that they don't have the same exact curve going on, as they do now. The second best thing to do, which would work almost as well, methinks, is to simply move the big back *into* the tree. That'd make it clear he's not sitting on it, and it'd also avoid the problem of the space between him and the tree creating that "negative outline" tangent you're worried about. =)
This is looking really good, btw. Though, like 9_6 said, I think your characters are losing to the background. Your characters are good too...they just don't seem to completely match the background, for some reason. It's kind of an odd feeling, honestly. = \
It might just be their poses. They're not super believable right now, to me. The guy's kind of aiming over the pigs head, especially considering the pig will be landing soon, and the pig doesn't really seem to be launching at him. Just kinda jumping in the air. The guy's flow is going straight right, and the pig's is just kinda scattered everywhere, vaguely going up and left. They just don't seem to be clicking together, or with the background. It's hard to explain, but it's like my eyes don't know where they're supposed to focus. No real direction in the picture.
Here's one of my favorite examples of awesome focal point/flow paintings, to give you an idea what I mean:

Notice how your eyes know exactly what you should be looking at. The boxers are brighter than the rest of the painting. There's clear visceral energy flowing between them. The guy on the left is EXPLODING into the guy on the right, and the guy one the right is obviously pushing back with all his might. The referee is like "holy crap! I want to stop this but it's too awesome! They have so much powah!" The coolest thing is that they both have clear directions they want to move in, and the focal point of the painting is right where their two paths cross - you can almost hear the two of them smashing into each other!
And one more example, from Hajime no Ippo:

So, sorry for the silly explanation, but I hope it helps.
To create more visual interest, I'd have one of the combatants lower than the other. Have the bird cowering back and to the ground in fear, while the guy keep his cool and aims up at the boar leaping from above! Right now your paintings is simply being read straight from left to right. The flow is boring. Which is a shame, because the art itself is very good! =)
Edit: Ah, another reason the bird looks weird is because his legs are pretty bright, even though the ground he's standing on is near black. The light source for the whole picture is a little vague to me actually, after looking at it again. The trees seem to be shaded from the back, and perhaps a bit from the left, but the pig and birdman both look like theyre shaded from the front!
Anyway, looking forward to some edits. This piece has a lot of potential, I think.