[**Updated since OP already updates their post**]
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Let's Compare the two
Excadrill
Pros:
Amazing Base ATK of 135
Good HP in 110 Base
Learns Swords Dance and Earthquake via Level-up, making it incredibly strong.
It's ability makes it a good partner to a Sand Stream Pokemon in Double Battles
Has 7 resistances, one of which a whopping 1/4 resistance and 2 Immunities, making it good defensively
Drilbur can be found very early on in Wellspring Cave
Cons:
Mediocre Speed of 88 Base
Pathetic Steel STAB, being only Metal Claw (1)
Mediocre Defenses, making it a little vulnerable against types it's weak to
Krookodile
Pros:
Good ATK in 117 base
Good HP in 95 base, making it a bit bulky
With 92 base SPE and enough training, it can outspeed and sweep
Has 4 resistances along with 2 immunities, making it decent defensively
Moxie is a good ability for a sweeper Krookodile
Gets Outrage, which is deadly with Persim Berry + Moxie combo
Cons:
Mediocre defenses
5 Weaknesses, most of which are common types
Intimidate is pretty much useless against SpA oriented foes
Learns EQuake much later than Excadrill
Since you don't have a Dark-type yet, then prioritize getting Krookodile to deal with Shauntal. Like Shroomish (A.k.a TastyMushroom) said, You might struggle against her and Foul Play hits hard for the Dragon-Dancers.
Krookodile @ Persim Berry/Any
Ability: Intimidate/Moxie
Recommended Nature(s): Adamant (+Atk -SpA)
- Knock Off/Foul Play
- Earthquake
- Outrage
- Stone Edge/Filler
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If you are fearless, however, go for Excadrill. Although outclassed by Krookodile in your case, it doesn't change the fact that Excadrill is still undoubtedly strong. If you're going with Excadrill, however, then you probably would need to hassle with finding another Dark/Ghost-type to counter Shauntal, unless you're fine with neutral match-ups.
Excadrill @ Choice Band/Scarf/Any
Ability: Sand Force/Sand Rush
Recommended Nature(s): Jolly (+Spe -SpA)
- Earthquake
- Swords Dance
- Protect/Rock Slide
- Iron Head (2)
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Hope I helped! ^^
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[Note 1: *Excadrill gets Iron Head through Move Tutor in B2/W2*]
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[Note 2: *You may be able to get Iron Head by transferring your Excadrill to B2W2, teach it Iron Head then transfer it back again to your Black save file. Please tell me if this is possible or not, though, since I've never tried it*]