So, I truly hope that there is some kind of counter card(s) to Lost World (perhaps a PokeBody that says "NO STADIUMS can be in play as long as ___random Pokemon___ is on your bench"
It's called Dialga G. By the time that you need to Deafen wall-off stadiums, your opponent will have 6 mon LZ'd. If you have 2 dialga G open, you will take 5-6 turns to KO them both... that should leave time to be called. Obviously, LostGar will be behind in prizes. That ISN'T including the possibility of Chomp and Poketurn. This just makes me want a Manectric EX reprint style of card even more... just to quiet people down about the topic
LostGar is a hard deck to play around. I've seen two types of builds for it: fast/consistent. Fast build is generally the worse of the two. Once you KO the 2nd Gengar in the fast build, you basically win. The consistent builds sometimes use Mew... sometimes not. The ones that do will hopefully (for them) LZ a gar T1/2 and then start going to town. These ones are very winnable if you have Dialga G in deck. You will have many prizes quickly, and if everything goes well, you will not have too much trouble. The other consistent build is a slow-and-steady Gengar build. These builds often get off around T2/3. This is the very hard build to play around (I have found). The constant Huntering (not calling it the American name) of Uxies/Mesprits/Gengars is just a real pain. They will usually get 3-4 Gengars out in play in a game... assuming the opponent is doing well. This matchup is a real big pain in the butt. Once again, Dialga G is an option, but it is less amazing because this build will be able to Compound pain pretty easily. I have seen it happen in many games. They have enough time and resources to do it. All three of these builds can be played around in a few ways. First off, I LOVE Cynthia and Judge right now for hand removal. I doubt we're going to see more than 1 Spiritomb TM per deck. I dislike it... and I'm sure a few others do as well. It feels very random and giving an opponent 6 cards (+ draw) is never a good idea. I barely feel comfortable giving my opponent a looker. Next, Junk Arm and Regice are your friends. The game will get to a point where you barely have pokemon left in your deck... and you'll happily Junk Arm/bebe/PokeComm/Regice them away. That means that the opponent needs to go Agro Gengar, or start using Hunter and hoping for the best. You will rarely ever need more than 1 attacker per game against Gengar... so just don't drop evos on bench and you should be relatively safe. Assuming they get 6 Hunters per game (Very possible), that will only give them 5 LZ'd mon plus your active remains. This is very hard to do and very situational, but I find myself putting Junk Arm, Regice, Cynthia, and Judge in things that probably don't need much. Techs that do one-shot gengar could work... theoretically. If anything, I'd say go Riot Honchkrow SV or Absol G X (yes, it exists). There are plenty of options to play against this deck...just work with it.
Is this deck over powered? I would say not. Is it BDIF? We'll see. Is it very good? I would say so. Another big problem I can see with it is match play. 75 minutes is not enough time for this deck to play a few full games. Since it barely takes prizes, it needs to be pretty fast to even win. It basically has to go 2-0 against a deck in top to survive. If it loses first game, it will probably have 40ish minutes left. It NEEDS to play 2 more full games to win. Seems very unlikely to me. Other decks (even VileGar) can try to take a prize against it in 3rd game to just win.
Once again, just my 2 cents. I have found a lot of this from testing. If you have not done so yet, try it ;D This game is not a science. You may get different results than me despite using the same variables.