Upon fighting the last two boss towers for the first time, I realized they were much more difficult than I expected. Just catching the legendary pals and pumping them full of souls wasn't enough to get the DPS to defeat a boss in only 10 minutes. I tried to look up videos of other people beating them, but all I could find were glitch videos or people cheating by turning the "damage from player" up to max and just casually melting the bosses with a weak weapon. So I recorded my method of defeating all the bosses.
My strategy (I show my build at the end of the video):
Breed a Sweepa with max attack passive skills (Legend, Musclehead, Ferocious, and Lucky), rank it up to 4 stars, and give it the maximum number of souls for HP, Attack, and Defense.
Feed the Sweepa every skill fruit I've found.
Breed 4 Swee with Vanguard (+10% to my attack), Stronghold Strategist (+10% to my defense), and Motivational Leader (+10% to my work speed, not really necessary, but nice) and rank them all up to 4 stars.
Ranking up pals increases the strength of their partner skills. Sweepa's skill makes him stronger for every Swee in the party. Swee's skill makes every Sweepa in the party stronger. This is why Sweepa is so strong.
Reset my stats and put as much as possible into attack. (5 into HP, 10 into stamina, 14 into weight, and the rest into attack. I definitely could've put more into attack if I really wanted to.)
Wear two Attack +2 pendants.
Eat cheeseburger and feed cheeseburger to Sweepa before fighting. (I don't know how long they last, I probably only needed two for all 5 fights.)
Increasing my attack as much as possible combined with the rocket launcher allows me to actually contribute DPS to the later fights. (I killed many many Jetragons trying to get the Legendary Rocket Launcher and never got it, so I just used the best one I could find, which was the Uncommon Rocket Launcher.)
This strategy seems time consuming, but is actually faster than a lot of other powerful strategies. There are a lot of combinations to make Sweepa so it wasn't hard to find pals with the passive skills I wanted to breed on to Sweepa. Since I had to breed so many I had plenty of them to condense for the rank. Swee consistently spawn in high numbers and are relatively easy to catch, so it wasn't hard to get a lot of them to condense. It didn't take long to breed four with the skills I wanted since there are only three (could do it with only two). It might have been better to give Sweepa Burly Body (+30% defense) instead of Lucky so he's a little tankier.
An example of another strategy that would take more time to set up (but is probably stronger) is to get four Gobfin ranked up to 4 star (Vanguard, Stronghold Strategist), a mount that gives you an elemental attack ranked up to 4 star (Vanguard, Stronghold Strategist, Swift, Burly Body/runner), and increase your attack as much as possible in every way. This would allow you to do a ton of damage with a rocket launcher. This strategy would take a lot more time because you would have to breed a different mount for every element (four if you just want to use them against the tower bosses.) and four Gobfins. Gobfins and all the mounts will be much harder to get 116 pals to condense than Swee.
Another strategy that might not be too grindy would be to breed a Frostallion Noct. According to the Survival Guide, Fusion Pals are stronger, so it should be stonger than Frostallion (a legendary). Since it's bred with a non-legendary pal it's much easier to get ideal passive skills onto it than the other legendary pals that can only be bred from two of themselves. Since it's dark, the only boss that has attacks that are strong against it is Axel & Orserk. You can use skill fruits to give it attacks that are strong against all the other bosses. I haven't done this, but I think it would probably be strong enough to beat all five bosses.
I started playing on Gamepass before getting the game on Steam. That's why my name is "Player941". It's also why I just got the achievements for defeating the first 3 bosses, even though I had already defeated them before. (This was my first time defeating the last two, though.)