I think Undyne has an AP advantage, as she took her house's explosion (100 million Joules) in base where she has 10 Def with no Damage, but Undyne the Undying was able to tank multiple hits from Genocide Frisk, and a weaker Genocide Frisk one shot Toriel,who has 80 Def,and Ringo has survived 68-92 million joules and damaged someone that can survive 291 million joules.
There's also Giant men surviving his own gun which is able to vaporize Meanies, who are like humans, and vaporizing Humans takes 291 million joules.
And if we only use the locomotive feat, wouldn't Undyne be too bulky? Base Undyne already took no damage from an explosion that is a bit above Casual Ringo, but that's base, Undying was able to take multiple hits from the human that one shot her and Toriel.
Well. The feat did leave them scorned iirc. Or some burn marks of them. It’s just they were mostly alright. Ringos feat would be very casual too. He did it with one arm iirc. He also has many hacks, which he uses most in character. Such as teleportation or duplication. As well, his own Danmaku. So even if do less damage, but he should be able to compete. The AP difference should hardly be much. Since both are so casual and almost the same level. At absolute most, you could say Undyne is maybe twice as strong. And even that is a bit of a stretch. Plus his lifting strength too, which is notably way stronger.