It's another Shadow Moses Island match. Huge terrain with multiple elevators that take minutes to ride up. Mega Man is long dead before the teams meet up.
So that leaves a 5 v 4, really a 5 v 3 since Rika is useless in a fight. Batman has his warp, but going 1 v 5 (really a 1 v 6 with Bahamut's help) against my team is a pretty bad idea. Additionally, since Liquid is getting info from REX's sensors, warping into my team's starting room won't work. He's better off sticking with Jim Kiryu and Leon.
The other neat thing about this terrain is that the only way up into the room that my team starts in is a slow elevator that gradually raises up to the floor of the room. It's a great place to get hit by REX's weaponry as well as Vivi and Cloud's spells. And before you ask "Why don't they leave to try to meet up with the other team?", REX is TOO BIG to get through the door on the lower level, so might as well camp the starting room than run around down a teammate.
Even if someone survives the murder hole at the start, the next thing they have to deal with is Zeratul. He's invisible, so he can get the jump on someone when the platform gets to the top. Rika isn't surviving the murder hole, so she's totally unaware of his existance no matter how many loops she has. The elevator is the only way up, so they can spend time looking for another way, but it doesn't exist.
If that wasn't enough, REX is a toughie to take down. Snake had to fire multiple rockets at its radome (that circle thing on the left shoulder) to down REX's sensors so Liquid had to open the cockpit and make himself vulnerable. The rest of REX's body was completely resistant to all of Snake's weaponry. Jiryu and Leon have worse weaponry here than Snake did. Batman's best gadget here is probably the remote hacking device, but that requires him to sit there and fiddle with it for a bit, leaving him open. The Batmobile doesn't help, since there's no way Batman would be able to get that thing in here. Without the Batmobile, the other team just doesn't have the firepower to down REX.
TL;DR: Mega Man is dead, we have the numbers advantage, there's only one way in to our starting room that's easy to camp, Zeratul is killing one person before they realize he's there, and the other team just doesn't have the firepower to take out REX.
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Virtue - "You don't need a reason to Boko United."