This battle takes place in a three-sectioned field- there is the inner ring, with the banners; the middle ring, with two bridges and a large pit that encircles the inner ring, making it hard to reach; and the outer ring, a large flat area that mostly doesn’t get used during battles.
As you enter, before you get a chance to act, Yoshihiro points out the banners, which are all at the corners of the innermost ring. Apparently, not only are the banners in difficult-to-reach spots, but any Pokemon struck while standing on ring’s edge- such as on a banner- will be knocked down into the pit that composes the middle ring.
This battle promises to be extremely difficult for a number of reasons. The most obvious one is, of course, the raw power that Yoshihiro’s army packs. While many of them have only passably good defense, the attack strength of these pokemon is very daunting- anything you can do to reduce the incoming damage, especially to Normal or Rock Pokemon, is going to help you massively. Second of all, you need to hold the four banners for five turns- this is going to be rather difficult as-is, since you can’t plant a Pokemon on a banner and use the HP-restoration from it to keep that Pokemon in play when any hit at all will knock you away from the banner. Third, there is a lot of area in the vital inner ring where Pokemon will be knocked out of that section of the battle if they’re hit at all, making any kind of stable tactic for keeping your opponent off the banners dicey. Finally, three of the five enemy Pokemon pack the Thrust power, which they can (and will) use to send your pokemon to the edge of the inner ring- and then into the center ring’s pit.
Fortunately, there is something easing this a bit- the enemy also needs to hold the banners for five turns to win, and with the ease of toppling them from the edge of the inner ring, it’s not hard to keep them out yourself, as long as you don’t make a mad rush for the banners and give them the chance to knock you loose first.
Ranged attacks are a useful thing in this fight, especially ones that skip one or two squares so that you can plant the attacker behind another Pokemon and create a more stable wall. Keeping control of the bridges that are the only way into the central ring is also important, as it can let you capture the banners much more easily. Flying and levitating Pokemon will go a long way towards fixing your problems as well, since they can just move right back into the innermost ring from the pit.
With the relatively small size of the innermost ring, abilities like Interference, Intimidate, and Lullaby are very good for cutting enemy offense, and if you can get across the bridge fast enough you can form your Pokemon up into a strong phalanx- especially since even Thrust or the edge of the inner ring can’t shove a Pokemon into a square that’s already occupied.
Make sure you make note of where the stairs out of the pit are located as well- most Pokemon won’t have the speed to get out and back into the fight in a reasonably short number of turns, and this only makes things worse if you don’t recall which way they need to go to get out of the pit.