(open mouth, insert foot mode)
Okay, so first off, I was the one that pointed out that it isn't in Ash's character to send out all of his Pokémon at once earlier before speed was equalized. Secondly, we also need to count Sans' personality as well. Normally, unless he was fighting someone with whom he has a grudge (such as Frisk/Chara), he won't put too much effort into it. Now, that being said, I do agree that since teleportation is able to be used by Sans, that gives him a huge advantage over Ash's Pokémon, some of the moves for Ash's Pokémon would help as well. For instance, Greninja has access to the moves Spikes and Smokescreen, which would hinder Sans' mobility and impair his eyesight, and Pikachu has access to Quick Attack, which would help him keep up with Sans.
However, I also do agree with the point that since Sans is so lazy, he'd probably impale Ash right there on the spot. However, he would also at least be impaired in his judgement to attack Ash directly due to his promise to Toriel. That isn't to say he will refuse to kill Ash, as stated in the SBA, but it would at least make Sans hesitate long enough for Ash to take advantage of it.
But, hey, to be frank, Sans impaling Ash right off the bat sounds a lot more plausible than Sans waiting until he grows too tired and have Ash's Pokémon take him out.