I feel like they are saving the Ash big tournament win for the finale of the anime (I'm not so sure because I don't watch it but I think that's their plan), but like you said Ash journey doesn't have to end if he wins the league. Maybe because of the backlash they will hand him the W next season

I am not the only one who likes to bully you.

If it says recommended level, then he's already playing on Easy. If he got a game over on Easy, then the Friendly difficulty should have been unlocked. So if nothing should help, he should turn down the difficulty for this particular fight as a last resort.

Enemies initiate sessions in the same way the party does: Striking a weakness with a skill. So the first obvious tip is to start the battle with a party that is not prone to allow enemy sessions. The second tip is to kill at least one or two enemies before they even get their first turn; at five levels higher, your brother's party should get to act first already. The third tip is to first target the enemies that initiate sessions. The fourth tip is to manage the party and see if it can still be improved on its skill sets; at this point in the game there should already be party members with access to the radiant skill Open Audition which allows them to join sessions while they are not in the active party, meaning that the action of one character in battle can trigger a total of five attacks; that can be up to 15 hits before the enemy party gets to do its first move in round 1 of the battle. The fifth tip is to use skills that hit all enemies, but it's necessary to pick an enemy as the main target that is weak against such a skill, otherwise there will be no session triggered. The sixth tip is that special performance skills can trigger sessions without hitting an enemy's weakness, so for example, Touma with his lance can still do severe damage to an axe enemy.

The point is, it shouldn't even get to the point that five enemies attack your brother's party because by the time they get to act for the first time, they should already be less than five. I think enemies respawn once the boss has lost half of its health, but they don't get to act immediately, so the strategy from the start of the battle can be repeated to decimate the enemies before they can do anything.

If he can't do it on Easy at five levels above the recommended level, then either his strategy is bad (which it has to be when all five enemies are still standing by the time it's their turn) or he managed the skills of his party so bad that it gets difficult. But by this point in the game there should have been hardly any occasions where he was forced to overwrite a skill, so he should be good to go as far as that is concerned. It's one of the trickier boss battles for sure because of the enemy count, but it can be done by getting off to a good start.

But would that new guy have any better luck?  I mean Ash hit Charizard with a powerful electric attack and other blows to wittle him down AND in the end had a really powerful Water Pokémon to take him on and STILL lost anyway despite every logical outcome being a win.  It's like the very gods are set agains the protagonist of the anime!  Or perhaps it's just intrusive writers who lack the imagination to keep the show going if Ash won.

But would that new guy have any better luck?  I mean Ash hit Charizard with a powerful electric attack and other blows to wittle him down AND in the end had a really powerful Water Pokémon to take him on and STILL lost anyway despite every logical outcome being a win.  It's like the very gods are set agains the protagonist of the anime!  Or perhaps it's just intrusive writers who lack the imagination to keep the show going if Ash won.

He should use those who aren't weak to the enemies' skills, because then the enemies won't be able to trigger sessions. However, I don't quite remember if you really have that choice at this point in the game because the higher priority is to have longer sessions yourself by putting the party members with the radiant skill Open Audition in the subcast, but not everyone might have access to it already due to their stage rank being too low. So it's likely that the active party will be Itsuki, Touma and Kiria by default because only Tsubasa and Ellie are able to join sessions as subcast members.

In any case, it's about gaining control of the battle by killing the enemies fast. Special performance skills trigger sessions regardless of the enemies' weaknesses, so they should be used if a character has no other option to initiate a session.