Ophie said:
Mew's Devolution Beam would work great too. Another substitute for Omastar would be Cradily (Legend Maker), whose "Devolution Wave" attack devolves as many of your opponent's Pokémon as you have React Energy cards attached to it.
Just thought I'd throw another option in there, there's always
Ancient Technical Machine (Rock) (Ex Hidden Legends), which allows you devolve all of your opponent's Pokémon 1 Stage for 1 Energy.
Note, Technical Machines where introduced in the E-Reader Series. They can be attached to any Pokémon (there are restrictions for certain TM's) and give that Pokémon an extra attack that can be used. At the moment, at the end of the turn the TM was attached, it is discarded. However (and I may be wrong, so someone else may want to correct if so), I believe in Japan from DP5 onwards (and thus in the next English set probably), TM's are not discarded after being used once.
Also, for that Omastar/Aerodactyl/Aggron Ex combo, instead of Aggron Ex there are 2 other choices. They are
Ninjask (Ex Deoxys) and
Ursaring (Ex Unseen Forces). Both of those are Stage 1's rather than Stage 2's, but their Basic-stopping Poké-Bodies are slightly less powerful than Aggron Ex's.
As for people still playing with older cards, I still do occasionally, and am actually trying to make a couple of decks, a couple of which combine some older cards with newer ones (I've got one like that which kind of works, but isn't exactly what I want). Me and my brother (who use to play and collect Pokémon cards) managed to get the whole of Base, Jungle and Fossil, though we are missing a few (5 I think) cards from Team Rocket. I would try and complete the set, but I can't be bothered to collect anything now (apart from promos, including old ones).
Ophie - hate to be picky on such a small thing but, technically, Supporters where introduced in the e-Reader series, but Neo Genesis did have a couple of Supporter-type cards, normal trainers that, after being used, stopped you from playing any more trainers during your turn.