I can't really help much for the second boss aside from recommending the bubble item (as seen in the video you included) linking to
this strategy from PSNP, but for the first (Dagron), I tried to time it so that after using a Super Monster Milk and stunning him the first time, hit him only 10 times, then button-mash like crazy after stunning him again. He's nearly impossible to dodge after speeding up, which he does when his health is low, so you don't want to button-mash him during the first stun. It might not help, but
here's the PSNP strategy for that boss, too.