Hasn't tackle been used plenty to beat rock types?
Maybe as a finisher but only when other moves have been used. Besides Excadrill isn't a rock type, otherwise Pikachu would have an advantage with Iron tail. But Excadrill is ground/steel Pokemon, even if Pikachu spams Iron tail and quick attack, it still should do so little damage to make a difference in the long run. Since Pikachu shouldn't be able to handle drill run and dig, and focus blast does full damage.
I guess we can compare Excadrill with a Pokemon with the same type: Steelix. Pikachu has never been able to beat a Steelix, Onix yes, Steelix no. Morrison's Steelix "killed" Pikachu because it was stronger, and resisted Pikachu's moves, Iron tail vs Iron tail (Steelix won), now Excadrill as far as I can recall can't learn Iron tail, but it does know metal claw, Excadrill should be stronger than Pikachu thus Metal Claw should be able to counter Pikachu's Iron tail if he used it. And Quick attack, might give Pikachu speed, but I don't know useful it will be to Excadrill since Excadrill probably wouldn't take much damage to it.
But to be fair to Pikachu, his opponent (assuming it's Excadrill) doesn't seem to be as fast as Snivy's opponent so that's something Pikachu has got going for him too.
I don't know about that because it doesn't take a lot of running speed to have good reflexes, and Excadrill has been shown to be pretty fast, it probably wouldn't take much effort for Excadrill to counter Pikachu's moves.
It's a different situation but the point is, even if Pikachu is at a disadvantage (against anyone), it has a chance to win,
Of course Pikachu would win, that's not what I'm saying, what I'm saying is Pikachu shouldn't beat Excadrill just because the writers are trolls and want to make impossible match ups. If Pikachu had at least one move that did full damage to Excadrill, then maybe Pikachu would have a better chance.
But if the writers want the battle to look realistic, Excadrill should win since it completely outclasses Pikachu. And like I said, Snivy and Palpitoad would make a lot more sense if they really wanted Ash to win.
The only reason why the writers would have Pikachu win, is to make some B.S. excuse to keep using it when it isn't needed (Pikachu beat Excadrill, it should totally be used against Swoobat over Roggenrola/Boldore), giving Ash's other Pokemon the short end of the stick.
I think a lot of people already suggested this but remeber in the ducklett episode where pikachu was battling krokorok? krokorok fired stone-edges at pikachu who repelled it back with electroball (a move that would have no effect on krokorok) causing it to take damage and blast off...maybe pikacu can use electroball to send back excadrill's focus blast or to throw him off direction when he's using drill run...of course this is all assuming that pikachu will even face excadrill...
I've said that previously. But Iris wouldn't be stupid to fall for it twice, and I'd find it hard to believe, that one focus blast would finish Excadrill off even if it was in the last leg of the battle. But it makes it a bit more believable.