in regard to:
http://www.toonworldacademy.net/t3552-garunix-vs-garunix

the answer is wrong.

since, wikia is accepted as a resource here, you guys are missing this statement:

This does not apply to effects that activate "During the ___ Phase,...", such as "Gladiator Beast" monsters. Effects like these do not form a Chain - instead, they activate, and are applied, separately.

http://yugioh.wikia.com/wiki/Simultaneous_Effects_Go_On_Chain

and because of it:
the effect to summon garunix from graveyard activates without forming chain with each other.

thus, the result would be:

turn-player activate garunix's effect to special summon himself from graveyard, and then resolve it.
turn-player garunix's activate his effect and destroy all monster in the field, and then resolve it.

non-turn-player activate garunix's effect, and special summon him.
non-turn-player's garunix activate his effect and destroy all monster in the field.

this is the result if both player know about how priority works, the result could be different depends on how far both player's understanding in regard to priority.

I did some research, and found you are correct.
Didn't read the effect of Garunix before answering the question, guess I should have.
'grats.