Well, many people say that asgore is capable of dodging attacks although it may not be true. Asgore in combat holds his trident directly in front of him it may be possible that he used the Trident to deflect all the hits. The reason he doesn't do it in his fights is because he feels guilty fighting a child, he's looking down on the ground through the battle and doesn't want to continue this fight. He knows he has to which is why he continues but a monster defense is lowered when they don't want to fight you, that's why you always hit him