So I'm kinda confused about how Reaver works. Does it only work after greenknee has summoned creatures? Or does Reaver have to be used first and then he can summon? Or am I missing the point?

Reaver is triggered at the end of the encounter that Greenknee gets Wounded.

The Summons is triggered if Greenknee uses Reaver.

But is there any surefire way to know when to use summon or Reaver

I'm not sure I understand the "surefire".

Reaver is triggered the Round that Greenknee takes any kind of Wound at all AND there is a Creature in play of Cost 2 or less. So, if you Wound him in Combat or any other way, and he's still alive by the end of the Round (Step 6) - Reaver will trigger.

If the conditions are not met (being Wounded and not having a Creature in play of Cost 2 or less) the ability does nothing.

At the end of Reaver you will notice a "+" symbol, that symbol represents that there is another ability appended to the condition of Reaver.

Below Reaver and following the "+" you will find Summon which is indented, as it dependent on the attribute above it, it does NOT trigger unless Reaver goes off.

I hope that helps, let me know if you had any more questions.

So reaver doesn't work if the wound was gained from a previous round? I think the rulebook says differently?

As long as he's Wounded at the end of any round, he will Reaver. There must be a Creature in play of Cost 1 or less to trigger the Reaver/Summon effect.

Speaking of this, I foolishly triggered it last night during a test!