Totally changed the card

This Planeswalker will be a Big Game Hunter, moving from plane to plane on an endless Safari to hunt the most dangerous creatures each plane has to offer.
Not quite sure how to go about relating that flavour as Planeswalker abilities yet though.

Decided to go with pumping creatures, Fighting and an alternate win condition emblem.
Hopefully he is relatively balanced.
Get him out, power up to ultimate & then drop another copy to use the -2 to help get to 10 trophy counters.

That's basically what Garruk did, before Liliana cursed him.

Hmm. Interesting ultimate. I find it odd that you can win in two turns with 10 mana, a Reassembling Skeleton and a Goblin Sharpshooter in play.

10 mana of which 5 of it black. That's a pretty unusual setup for a planeswalker. Any alternate win condition will be gamed; this one doesn't seem especially more breakable than any other. (I'd go for Varchild's War-Riders, Hunted Troll and Simoon myself :P )

OK changed it to "whenever an opponent's creature....." to stop the shenanigans with your own creatures.

I wanted an ultimate that reflected the flavour/background of the character concept and AFAIK there hasn't been a true alternate win condition Planeswalker ultimate yet so that seemed like a good way to go.

It's tricky. I like the idea, but I do expect a bunch of grumbling from players who are all like "but my opponent doesn't play many creatures! And I have ultimate up! I wanna do an ultimate!" Not like all ultimates do anything anyway. Who cares if all your permanents are indestructible if your opponent plays a prison deck?

But that may mean that 10 creatures is too many creatures. I mean, Ultimates are supposed to do something that makes the game feel like it's already over n'est pas? This ultimate feels like a five or six casting cost enchantment that you ideally want to play on turn five or six, and it changes the way the rest of the game plays. Maybe you should cut the required number of trophies to a number closer to five. That should set the timer ticking. In theory, this means that the ultimate makes you come close to auto-winning in large group games, but, if people see this coming, I think everyone at that group game will end up attacking the Planeswalker anyhow. If they can't nudge occasional loyalty off him... well, the game should be over anyways.

First ability: I like this, but what if it gave a +1/+1 counter rather than UEOT? Then it complements the second ability more. I like planeswalkers whose plus abilities make their minus abilities better.

Second ability: Simple is good. I especially like how the new action word fits flavorfully.

Third ability: I agree with jmg, it should be fewer than 10. Say you play this turn 3 (it's in a green deck): as long as you never fight with it and no damage is dealt to it, the soonest you can ultimate is turn 6, at which point you have to kill 10 more creatures. It's not enough advantage to be worth using.

Ah I see a little more information is necessary here. You see, Nasun doesn't work alone. He has underlings that help (as all good Planeswalkers do) and some of those underlings will also be generating Trophy counters for you. So, by the time you hit his ultimate you may have anywhere between 0-say 5 or 6 trophy counters already accumulated.See Big Game Hunter for possible implementations of this.
I know this kind of thing is hard to anticipate but, taking this new info into account, does the 10 counters now make sense?

I can see where you're going with this now. I guess I'd have to play these to see what the right number would be.

I think that I will have to lower the number as I am thinking of making Trophy counters a genuine alternate win condition similar to poison counters. If I do the ultimate would reduce the number needed to win.
On the other hand, if I don't I may still tweak the numbers or change it up a bit. Just not sure of another way to convey flavour-through-ultimate so well.