Talon Ultimate can stack on creeps.

[kil201]

To test:

1. Turn on No Cooldown or buy refresher.
2. Hit the wave with first ultimate (creeps should be low hp after hit)
3. Right after use your second ultimate in this wave (not slower than 4 sec timer)
4. For all creeps you killed after first hit of your ultimate you will get +5 to spirit power.

 

[kil201]

There video showing it

 

[kil201]

After some test. You need just hit the creeps and just get them with your LMB and you will get a stack from creep