The Blu-ray Release for Super Sentai Saikyo Battle!! had included the description of whole story and it reveals the Last Boss of this.
First let's start from the part of the story guide which mentions it.
Super Sentai continue to attack Gaisorg who wants to challenge & fight the Strongest One in the Universe. Meanwhile the true meaning of hosting Super Sentai Saikyo Battle comes to light gradually.
Using the battle of Super Sentai, they were planning to revive Dai Satan in order to destroy the whole universe. The end of the Universe is getting near. Will Super Sentai be able to save the universe by bringing their Hearts of Justice, together?
Dai Satan was the Last Boss of Kyoryu Sentai Zyuranger and he appears only as a head. It is not known whether he'll actually revive in this or it will end up being an attempt to revive him.
It is not known why he is chosen as the Last Boss but the series Zyuranger is considered as a point where Heisei has started in Super Sentai even if it is a year later. Another thing could be them having some connection to the Ryusoulger.
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Ryusoulger Say Super Sentai Saikyo Battle!! is Going to Start