Unfortunately, Yanmega isn’t a really reliable option. It’s speed and Special Attack May be high, but it’s weaknesses and other low stats like Special Defense might be a danger to him.
I see you want a Pokémon to counter strong Psychic and Dark types, instead of a Bug type, which not take Drapion? This Pokémon incredibly helped me with the league.
He’s got a nice movepool, nice stats, and has only one weakness, is immune to Psychic type moves, resists Dark type ones and can even counter them!
So, if you’re interested, Skorupi can be caught in the Great Marsh in Pastoria city, and here’s a moveset for Drapion.
Ability :- Sniper
Crunch/Night Slash - Drapion gets Crunch at level 49, while a juicier option is Night Slash, which, paired with Sniper can OHKO easily. Drapion can be taught Night Slash as breeding move, and his parents can be... Yanmega himself!
Cross Poison/Poison Fang - While you’ll have to wait longer to get Cross Poison (level 59), Poison Fang is available quite early.
Ice Fang - Coverage against Ground types, his only weakness, and a few other types as well.
X-Scissor - A powerful Bug type is all what Drapion needs to sweep Dark types, while taking little damage.
NOTE :- In my moveset, I mentioned two moves that have higher chance of critical hit, if you are willing to breed Skorupi (for Night Slash) and level up to 59 (for Cross Poison), you could make him hold Razor Claw, for higher chance of critical hit.
With this trick, and with Sniper, each of both moves has 1/4 chance of critical hit, one critical hit inflicting 3 times more damage as a regular move would do (due to Sniper). Imagine how he could wreck Lucian’s team( Psychic type) in Elite Four.