Syndicate Hideout is the seventh (A) stage from Streets of Rage 3. This is the final stage on the good ending route; if you're playing for the second bad ending, you'll instead end up at Stage 7-B.
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Before stepping into the hideout, you have to walk through the forest of Death and be met with blades, spikes, bikers, ninjas, grenades and van of traps. It's quite the onslaught, requiring a lot of skill to survive.
After making it inside the factory, you'll be met with extremely weak duplicates of, Robot X — the bad ending final boss if you play on easy mode. You will then encounter numerous robots and armed bodyguards...on conveyor belts! You should also be wary of the lasers coming from the walls. (These lasers can harm the enemies too.) A jet clone will appear at the end as well.
When you make it through this arena, you'll find yourself on a conveyor belt constantly pulling you towards a deadly green laser in the background. Further in the background is Dr. Dahm himself, controlling a mechanical arm. Destroy it to defeat him and move on to the final area to confront Mr. X...or rather, his brain.
A cutscene will trigger, where Mr. X will be furious at the heroes for destroying his robot factory and defeating Dr. Dahm. When told to surrender, he will reveal several bombs planted around the city and set to blow by his "loyal followers." After being asked by Zan if he really plans to run the city from a glass phial (vial), Mr. X will then summon Neo. X — the true final boss.
In order to get the good ending and save the city, you must defeat Neo X before the time runs out. (If you're playing on Hard, this is an almost impossible task, as Neo X will have eight health bars.) Failure to defeat the robot before the time runs out will result in the third and final bad ending where Axel is unable to defuse the bombs in time, and the city is destroyed.