I guess first off, I feel kinda bad for the creator of this special seeing as how many people are hating on the story. He got handed an invincible fighter and told to make a conclusion with said fighter.
On the contrary to what I just said, I do feel like they wasted a perfect opportunity to make a good conclusion for Broly. He's the Hulk of DBZ, and even he ain't invincible. Take World War Hulk for example, it's about Hulk going commando on all the Marvel heroes of that era, but even he doesn't challenge all of them at once. He picks them off in small groups or even individuals at a time, and even THEN he gets injured.

DBM Broly's way too over powered, so let's scale him down to just below Ultimate Gohan's level. With that, we can make an entertaining story. Here's how I would've told it.

Let's say this story takes place 5 years after Buu dies. It goes better with the power scaling, fills in a HUGE needless blank period, and I don't see Broly screwing around for over 10 years since last time, he headed straight for Earth.
Broly still does an Aerial strike which ravages the Earth, leaving only Goku, Goten, Trunks, Vegeta, and Gohan alive. As everyone's awake on impact, they take minimal damage except for Gohan who was asleep, and gets gravely injured.

Broly first heads for Vegeta who's coincidentally with Trunks. The 3 of them fight with Broly having the upper hand, Vegeta tells Trunks to flee which he does, but shortly afterwards Vegeta gets killed only having given Broly a few small bruises. Before Trunks can reach any of the Z-Fighters, Broly catches up and brutally slaughters him. There goes Vegeta and Gotenks.
Goku TPs to Broly, leaving Goten behind since he's not at that level yet, goes SSJ3, and fights Broly, with a heavily injured Gohan rushing in to avenge Videl/Pan. The two of them over power Broly and start to do some heavy damage on him, but before they can finish him, Goku runs out of Ki (Ala SSJ3), and Gohan succumbs to his injuries, allowing Broly, who won the war of attrition, to kill them in the most brutal way imaginable, finally satisfying Broly's long quest.

Somewhere else, Buu absorbs Piccolo and a frustrated Goten, and heads towards a very injured Broly. The two duke it out, with Broly's 'invulnerability' and Buus regeneration giving for a lengthy fight. Every time Broly destroys Buu he regenerates, and Buu's attacks are hurting Broly, but not enough to put him down. Buu uses the SPC, Broly dodges, seeing that that would've killed him otherwise, and then out of fear and frustration, he launches a volley of Kis that destroys him. As he regenerates, he keeps getting hit by the volleys, until he (Buu) runs out of steam, and gets obliterated once and for all.

This way Broly still comes out as a mighty Hulk levelled fighting machine, but he has a believable struggle before he comes out on top.

Kaioken Maximum sounds weaker than just using the ordinary Kaioken, since the entire point of Kaioken is to multiply the facilities and current abilities of your entire body, as opposed to just drawing out it's existing full potential. It's no different now from Ultimate Gohan, or any other kind of form or external power that draws out one's full potential like Guru. You're not really pushing the limits of your body.

Kaioken as form on the other hand is meant to push the body's limits. That's why the price of using it is your body getting hurt by it, byou're physically overclocking your body to work as many times as hard as the multiplier you're using is. It's like overworking and pulling a muscle, or even tearing one. The return however, is that you're twice as strong. And it keeps going up with the number, to x3-4, to x20, and if one is willing to play with fire, possibly higher.

Because that's the entire point of the form. It exists to give weaker fighters who are willing to risk it all that extra edge they need against a marginally stronger opponent, or even the playing field with foes who are much stronger than them. Kaioken Maximum conceptually just takes away what made Kaioken good.

Shinku was saying: ok, HOW EXACTLY does buu lose this fight? they cant even say he dies from the earth blowing up cause buu is capable of surviving that

I'll tell you the end even if they demand me ...

well, finally ubb shows up and broly takes him hostage ... ultrabuu can't attack him and brocoly with a super ultra mega blast completely disintegrates him. remember that buu can be destroyed with a sufficiently powerful and fast energy bomb.

then broly takes a well-deserved nap and freezes from the cold of space.

BassMaster516 was saying: Gogeta is their only chance, but I guess they don't have time to learn fusion. They should teleport to King Kai's planet and think this through. Things can't really get worse for the earth anyway.

Since Salagir has been making movie villians canon they should know the fusion dance. Janemba should have also happened in this universe, so they should have already became Gogeta years ago.