The pokemon have logical brains that can formulate thoughts high enough to make strategies(shown by pokemon communicating in "Island of the Giant Pokemon"), so why wouldn't they do one of the following by teaming up?
1. Psychic type pokemon use their Psychic powers to lift the lions into the air. Dialga freezes time where they are, and the pokemon can attack until they run out of energy, and then rest until their ready to attack again. Even is you where to say Dialga would get tired, why wouldn't things like Caterpie use hordes of String Shot so the lions can't move when they get out? Or that fire type pokemon wouldn't work together in making a magma pit under the lions, so that when Dialga gets tired and have to un-freeze time, they all fall into a fiery pit and melt?
2. All the pokemon climb onto a mountain, and Palkia makes a rift around the mountain, so that when the lions attempt to climb up they get teleported back to the bottom. Therefore the pokemon with long-ranged attacks can pick them off for as long as they want. If Palkia gets tired, Dialga could stop time around him so that he can't react to his tiredness.
3. Kyogre makes the whole earth a sea, while Groudon makes an Island that the pokemon can stay on, so they don't get tired. The lions drown while the pokemon are fine. These are the pokemon's natural abilities, its what they do, so why would PP limit a pokemon from doing its purpose?
4. Darkrai puts the lions to sleep, and the pokemon rush in and kill everything
5. Malamar and some other Psychic type pokemon can control minds( "A Conspiracy to Conquer!) So there's a bunch of Lions who are attacking each other. You could argue that the Lions need to see the pokemon to be mind-controlled, but with Eternatus' unlimited Dynamaxing, all the Lions would be able to see the Psychic pokemon, which means they would all be controlled, and then forced to kill themselves
6. Pokemon such as Tyranitar and Hippowdown summon a Sandstorm, and since we can discuss items lets say the have Smooth Rocks. They Lions can now barely see and are getting chipped away slowly. Sand Rush users such as Excadrill, Sandslash, and Lycanroc then run through and causes havoc among the Lions. The Lions most likely won't be able to focus and the Pokemon can win. Same thing for Swift Swim, Chlorophyll, and Slush Rush users, as well as Surge Surfer A-Raichu
7. Its been said that a large portion of pokemon are weak and would auto-lose. But a large majority of them get Sand-Attack, so who's to say they don't from a giant group Sand-Attack so that the lions can't hit anything?
8. Water pokemon can sit in the water indefinitely. They can just sit in the water and attack until the lions are dead. If they get tired they can rest in the water without worrying about attack. While Lions can swim, the only do so if they have to, and they cannot do so indefinitely, so eventually they would just drown.
9. In several episodes its shown the Pokemon can dodge attacks using Agility. Why not avoid the lions attacks doing so?
Other arguments:
People say Macargo and other incredibly hot pokemon would eventually be overwhelmed, but at the temp. anything that gets close is melted, so how would the lions pile on and overwhelm it? (Macargo def could not solo the lions though, its a snail it can't move fast)
Ghost type pokemon can float without effort, and they can go through things, so they cannot get tired of it. Same argument as flying pokemon, but better.
Why do the Lions have typeless attacks as well as being typeless? Why wouldn't they have type based attacks they could do such as Bite, Crunch, Snarl, Slash, or Roar? That alone makes it a bit unfair.