How do you beat mossdeep gym leaders tate and liza?

Answer 1

have some really high lvl Pokemon (lvl 50 or over)

if you can get a psychic Pokemon (alakazam or gardevoir) and teach it shadow ball (TM on top of mt pyre inside)

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I read that the minimum lvl could be 36. I lost once to the leaders and am now almost to lvl 36 with these two Pokemon, lombre (wtr/grs) and sharpedo (wtr/drk). If you're playing Emerald, which adds Lunatone and Solrock to their team... water, grass, dark and ghost moves are highly effective.

My lombre knows Solarbeam (found in Safari Zone, past upper left pond), Giga Drain (from a grass loving trainer as long as you have a grass type) and Surf and my sharpedo knows Crunch and Taunt (taunt being good for all those status boosts that your foes will use to max out their defense)...

I prob could do better with Sharpedo's dark move, I will check that out before I try a second time.