Actually Goombario was probably the best partner, as long as your enemy/boss was not spiked/allowed Goombario to hit them. Like basically what you could do, for about every boss or enemy if you wanted, was basically you buy a bunch of Stone Caps (or if you have Repel Gel, same principle). You keep Mario invincible/invulnerable for as long as possible, and have Goombario Charge up. Then, when you feel like it, you use Multi Bonk. Most Bosses will be OVERKILLED on that single Multibonk if you land just 5-6 consecutive blows, each doing over 10-20 Damage lol. You can beat any of the harder bosses by doing that, like the Final Form of the Master, Kent C Koopa, Final Bowser, etc. Because almost every enemy solely targeted Mario, even Bosses who rarely had Dual Attacks, there was no or little risk of Goombario being hit and slowing things down. So you just sat there, invincible, growing more powerful as each turn passes lol. Honestly, once you realized this, it made any boss battle in the game once you got the Ultra Stone a joke. Lava Pirahna, Huff N Puff, Crystal King (when he copied himself, you would have to use a Thunder Rage or Fire Flower or Star Storm as Mario, and then on the real King use Goombario's multibonk), you name it. Nothing would be challenging anymore, bosswise, once you got that. I sometimes would do it, but it was more fun to do boss battles "normally"