Hi everybody!

Im in Mt Coronet, and Ive gone all the way inside, to a place where there is a galactic grunt that says "Im hopelessly lost! Please don't pay attention to a lowly grunt without pokemon!"
Im stuck here and I don't know what to do.

:)

The cave that's behind him is to get to the bottom floor of Mt. Corenet, he leaves after you defeat or catch Palkia. Go back outside and climb up the rock wall and explore the rest of Mt. Corenet.

Thanks

Should I use the masterball or leave it for one of the lake trio?

Palkia is actually one of the easiest legendarys to catch. You should save it for either Cressilia or Mesprit. If you can catch Cressilia or Mesprit without a masterball than do. Cressilia and Mesprit flee battle and run around the map like Latias and Latios do in Ruby/Sapphire so Masterballs help extremely. But if you manage to catch them without master balls, great because there are a lot more legendarys to use the Masterball on.

How much Hp does palkia have?

From 150 to 197, depends on nature of Palkia. I would suspect around 180 hitpoints

So could I finish him off with endeavor and a lvl 61 infernape?

Yes you could, make sure you use endeavor when he's almost dead because Palkia can use Water Pulse and would take out Infernape in one hit. Also remember that even though it's at one hp you won't catch it on your first try.

Lets say that my pokemon has 30 hp, and it uses endeavor, palkia would have his hp cut to 30 right?

He could have his hp cut all the way down to one with endeavor. Endeavor will make sure your foe has at least one hp left. But don't worry so much, if you save before you begin the fight you won't have anything to worry about.

Right now, I'm in the 2nd snowy part in mt coronet. The part where you have to turn left or go straight by moving a stone, what route do I take?

If you want to get all the items on Mount Coronet, go in both directions. Otherwise, I think it's a left.

Where exactly is palkia in Mt Coronet?

Just make sure that if you enter a cave, if the cave has an exit with the white light showing and it wasn't the one you went through that means your about to get to the peak.

Palkia is at the very top, and you will be in a cave going up a couple stairs and you will see an exit with the white light showing.

Right now I'm going all the way up some stairs, I just went inside the mountain again, and some stairs are in front of me, some new grunts appeared, am I on the right track?

Yes you are, the battle is coming up.

An ultraball would be fine right, or should I use a duskball, or a timer ball?

Ultraballs are better. Timer balls if you exceed 20 turns.

Thanks, Im just about to defeat the last 2 galactic grunts, I'm in the spear pillar now