>>4120
Are we sure Frisk isn't just beating the monsters with the gun; pistol whipping them into submission, one might say?
And the tomato is simply a joke, it's not indicative of Frisk's actual strength. It's like Mad Dummy. If you have lots of LV, then Frisk attacks him full force and knocks him backwards. I think it's more Frisk not wanting to actually destroy the tomato and risk ruining the counter of the monster who was kicking his ass a second ago and has a fiery temper.
And if Determination does determine damage, then surely Frisk would be walking around one shot-ing monsters. Or Flowey would be a bigger threat to the monsters in the underground and simply take the souls from Asgore in the first place.
>>4123
Frisk would probably find him half melted and realize that the Doc was experimenting with determination and creating Amalgamates to invade the surface.