Ok, I've never heard this one before, but I do know that after you do this glitch and you kill other wyverns, after you keep attacking them it'll look like you're still attacking it even though it's dead. (with the blood flying out every time you hit it) Try attacking it with your sword a lot of times, ( a strong one with good sharpness) and see if that does the trick. You might have just found a glitch to make the Garuga even easier, although I think you need to write a guide as to what armour, weapons, items, etc. you need to actually defeat it, since you're one of the few that actually have. Well, I hope this works, again I've never heard of this so I'm not sure what will happen. Also, you need to repeat the pattern when doing the glitch... Spiderweb (won the battle), Kill it in less hits (won the battle) [or] Fail quest (lost the battle), Spiderweb (won the battle), etc. I think that the game makers of Monster Hunter should've just made the Garuga 10x easier instead of making a cheat to beat it. That way it would still be a hard as **** monster, but there wouldn't be any problems with doing the quest later.
***ALSO!!! Each time you do this quest, the garuga is GUARANTEED to get BIGGER! This not only gives you more items (usually), but makes the garuga HARDER TO BEAT. Once you do this cheat, the garuga will NOT GET SMALLER no matter how many times you do the cheat. Like I said, I haven't actually killed it, so i don't know if that'll reset the garuga's steroid-like size.***
Good luck with the problem, hope I helped.