How do you beat the pokemon league?

Try getting the legendery Pokemon first then you should be able to beat the elite 4 pretty easy

Did you captured Groudon? Do you have a Masterball? You need these 2 things!
The Masterball can be found in the Magma Hideout and Groudon can be caught with less than 50 ultra balls. You should buy almost 50 full restores with a possible 37 full restores left. You can also get a lot of money and get you Pokemon stronger by defeating Gabby and TY.

Also, The best Pokemons to have are:
Groudon- 53: Earthquake; Overheat; Dragon Claw and Thunder
Swampert- 55: Earthquake; Surf; Dig and Blizzard
Armaldo- 54: Brick Break; Earthquake; Slash and [Any rock-type move]
Gardevoir- 53: Psychic; Shadow Ball; Shock Wave and Ice Beam
Altaria- 56: Perish Song; Aerial Ace; Dragon Claw and Dragon Breath
Crawdaunt- 54: Guillotine; Waterfall; Crab Hammer and [Any rock-type move]

PS. I used them and I beat them in less than 30 minutes and also don't use Guillotine on Steven's Pokemons and use Perish Song on Metagross if it's the only one left alive.

The way I bet the PK LEAGE I had a sceptil Lv 62 with dragon claw, leaf blade, earthquake, and arial ace. Medicham lv 59 high jump kick, recover, fire punch and ice punch absol lv 60 thunderbolt, ice beam, slash,sword dance. Walrein lv 61 iceball, surf, sheer cold waterpuse. Groudon lv 64 earthquake, fireblast , solarbeam, eruption and finaly slaking Lv 63 Slash, focus punch, thunder and faint attack.

I used level 77 sceptile, leaf blade, cut, absorb and false swipe. Level 81 swellow with aerial ace, fly, agility, wing attack. Level 92 pelipper with surf,aerial ace water gun and wing attack
Level 93 raichu with thunderbold, thunderwave, agility, any two other electric/fight type oves and the other Pokemon I used were tentecruel and latios/salemence/combusken/empoleom! I/d just use Sceptile,Swellow,Pelipper,Raichu,Empoleom andLatios wll thats how I beat the league in gameboy ruby

Im saying something agenst the last guest you can't get empoleon on ruby it's only avalible on diamond, pearl or platinum as a starter. I am also haveing problems defeating them. Im trying to level them up as best as I can you might want to try this to.

Just get all your Pokemon level 60 + up
I beat them with gardevoir groudon blaziken salamence swellow and aggron