Well, problem is Bowser is a no-sell above 23 Gigatons in durability, and scales pretty far above it in AP, so any hit he lands here is gonna hurt to say the least considerijg Tohru is about 10 Gigatons, especially since both seem like brute force fighters.
And, while Tohru has two good hax, and one big utility for use against Bowser, the hax aren't exactly useful due to Bowser being able to rather easily return from BFR and having been brainwashed like twice now and still fought as normal...
As for the Invisibility... you can punch a tank all you want, but if you aren't even scratching the paint, you aren't winning, and in this case, Bowser has AoE Shockwaves he uses IC.