You’ll climb some stairs and then find a pair of Ace Trainers waiting by a few stalagmites.  You -can- face them one at a time, but there’s really very little reason to.

Battle: Ace Trainers Ray and Cora (4680 Poke)
Opponent (Ray): Vibrava Lv 39, Camerupt Lv 39
Opponent (Cora): Grumpig Lv 39, Drifblim Lv 39

Keep heading East and you can find a scientist who wants to know all the pokemon on Reversal Mountain.  Thankfully, you’ve probably encountered all of these (though not necessarily while here) and your Habitat List should be full for the area.  If it is, the scientist passes you TM54 False Swipe.  False Swipe is a 70-power Normal-type move that has the special attribute of never knocking out a pokemon it hits.  This is of especial use when you’re trying to catch something, as False Swipe will prevent accidents.  Sadly, the move is useless on Ghost pokemon, so you will need to find other ways with them if you’re having trouble in your capturing endeavors.

Continuing East will lead you to an opening in the wall, through which you can find another tunnel that leads on to Undella Town.  Since you’re not heading there yet, so instead head South from the scientist.  A stalagmite on the East just after some stairs holds an Elixir, a useful item that restores 10 PP to each move the using pokemon has- vital in long dungeons or the Elite Four battle series.

Continuing down the tunnel brings you to a passage leading down.  Go through it and collect the Water Gem from the stalagmite here, then head West down the stairs to face a Pokemon Ranger.

Battle: PKMN Ranger Lewis (2280 Poke, Persim Berry)
Opponent: Watchog Lv 38, Grumpig Lv 38