The final Gym Leader of Lunatic Crystal is the former Kanto Champion Blue. Unlike in vanilla GSC where you can challenge him right away, here you will have to defeat all the other Kanto Gym Leaders before you can fight him in Virdian City, similarly to HGSS.
Blue does not have a type speciality and his team consists of a very powerful and diverse team with some of the best Pokemon of their respective types, namely Tauros, Alakazam, Jolteon, Kingdra, Tyranitar and Venusaur.
Tauros is his lead and it's an all out attacker with STAB Return which is boosted by a held Pink Bow, Rock Slide, Earthquake and Megahorn. With 110 base speed and a high attack stat Tauros is easily capable of doing a lot of damage, especially with the great coverage that it has, not even Steel or Rock types are safe here. The best anwser here would be a fighting type like Machamp or Heracross as Fighting is Tauros' only weakness, but keep in mind that a crit Return will KO here. To be safe paralyzing it is a good choice.
Alakazam is next and with amazing speed, incredible Special Attack and a STAB Psychic boosted by Twistedspoon it's capable of doing a lot of damage and can easily tear apart tired teams. It also has Ice Punch and Fire Punch for coverage, so Steel types aren't the best answer here. Alakazam can also spread paralysis with Thunder Wave. Alakazam is strong, but as per usual it's very frail and a strong Dark type should be able to easily deal with it as long as it's not weak to any of the elemental punches. Tyranitar, Houndoom and Feraligatr are all good answers here.
Jolteon is Blue's fastest Pokemon and with good Special Attack, STAB Thunderbolt and Hidden Power Water for coverage it's another big threat to watch out for. Just like Alakazam it has access to Thunder Wave as well and it can also use Swagger to confuse your Pokemon. While Jolteon is frail it does have a held Focus Band which can let it survive moves that would otherwise knock it out. Hidden Power Water makes countering it with a Ground type harder, but a Pokemon like Quagsire can deal with it rather well. Miracleberry here is a good idea as Jolteon has two different status conditions it can spread.
Kingdra is tough to handle, with good all round stats and an incredible defensive typing that only leaves it with one weakness. STAB Outrage will do a lot of damage and it also has STAB Surf and Ice Beam, as well as Toxic to badly poison your Pokemon. Paralyzing or putting it to sleep isn't easy either as it has a Miracleberry to recover from status, which also lets it heal the confusion from Outrage. The best answer here would be to exploit its' Dragon type weakness by using a Pokemon with Outrage of your own, using a Pokemon like Blastoise that resists all its' moves or a dedicated special wall like Blissey.
Tyranitar is a Curse Sweeper with a held Quick Claw which makes it a scary prospect to face, especially with its' high bulk and incredible Attack and Defense stat. It has STAB Rock Slide, Earthquake, as well as Thunderbolt for a special option that is an answer to Water types. Just like with Tauros the best answer here is a Fighting type that can outspeed it like Heracross, Hitmonchan, Hitmonlee, Hitmontop or Primeape. Typhlosion is risky here as Tyranitar can easily KO it with Earthquake and Submission isn't a guaranteed KO. Curse can also let it survive fighting moves after a defense boost. Putting it to sleep or using a Pokemon that Tyranitar would outspeed anyway like a slow and bulky Grass type isn't a bad choice either.
Venusaur is Blue's last Pokemon. It can put you to sleep thanks to Sleep Powder and it has passive recovery with Leftovers and it has STAB Solarbeam, STAB Sludge Bomb and Earthquake as offensive options. Earthquake gives it much needed options against Fire and Steel types. Venusaur can be a problem if it manages to get off Sleep Powder, but a strong Flying type should be able to take it, Skarmory completely trivializes Venusaur here.
Overall Blue is one of the hardest fights of the game with fantastic type diversity, great offensive options, a lot of status spreaders and a combination of speed, power and bulk. To win your team building will need to be on point, only a strong, balanced team will succeed here.
One thing to note as well, after beating Blue you gain access to Rare Candies is the Indigo Plateau which are sold for 10 Pokedollars each. Make sure to grab them as they will spare you the grind for the final battles of the game.