Anyone uses Hidden Power? Does it use S.Attk or something else?

It uses special attack, but the base power of the move varies just as it's type. There's a man on the pokemon center on Mistralton City that tells you the type of the pokemon's Hidden Power but not it's base power.

All in all I rarely used Hidden Power given that even at it's peak power, 70, it's not that big of a move.

Well, it could be useful to cover your weakness.

I.E. My Lilil is a grass type and hidden power is ground type. The ground type would be useful to cover its weakness without have to switch-out. But I read that the stat that is use is not based on S.Attk.

On prior gen it depended on whether the move was Physical or Special to determine whether Attack or Special Attack would be used, but since gen IV, the move has always been considered Special. 

I don't see how Hidden Power would cover for much weaknesses, especially if you get the lower end of the base power spectrum, given that you wouldn't get a STAB from it and at 30 base power, a ground attack against a Fire wouldn't do much damage, even considering being super effective.

I decided to battle the Elite Four, however I need more training. I caught Volcarona before hatching a Larvesta egg, have you guys also used this format.

Yeah, you can catch the level 70 Volcarona on the bottom of the Relic Castle and use it, though Volcarona just doesn't seem to be that good of a fighter to me. 

Yes it's a great sp.attack sweeper and yes it has amazing boosters like Quiver Dance and Silver Wind, but if your opponent uses Archeops, Gigalith or anything that can pull good damage with Rock attacks, Volcarona is pretty much toast. 

Heck, Volcarona was Alder's easiest pokemon to fight >_<.

Not counting the set ones (ie Gyarados, Pichu and the Legendary Beasts) I was rather unlucky until Gen IV. Ran out of Pokeballs on Gold so missed the Red Gyarados, did run into a shiny Sableye on Sapphire which became my sole shiny for several years. Had a shiny Gyarados (random encounter) and Nidorino escape on LeafGreen, before bumping into a shiny Kricketune a long way down the line, which also nearly escaped but this time had a MasterBall. A few months later got a Snover which didnt cause too much trouble. Found if you just forget about them, theyll pop up as a pleasant surprise.

ive been playing pokemon since gen 1, never encountered a shiny

That's a pretty feasible possibility, given that you could go over 40K pokemon encounters without finding a single one (based purely on statistics). 

Ive finally realised how to get past the quick sand in the Relic Castle and Im think of going back there to get the fossils before trying to capture Zoroark. Im assuming I can only get one fossil (someone correct me if Im wrong). I know Archeops is generally the better of the two but was wondering if I were to get that, does anyone have a Titouga (egg) who would want to trade?