Oichi makes a good case for training Starly, as it is functionally almost the exact opposite of Jigglypuff, being a highly mobile and somewhat fragile ‘mon with a strong offense. Being a Gold Medal Pokemon for her doesn’t hurt, and especially in Illusio it can be a welcome change to have her ready this ‘mon.
If you have a shot at bringing a Gyarados to this fight, that’s not a bad idea either. Outside of those options, you’ll want to make sure you avoid Fighting-types- except Scraggy. Chances are you have one or two Warriors who are well-matched with them, and their combination of Faint Attack and Dark-typing makes them a good choice for secondary ‘mon here, despite their mediocre performance otherwise at this point in the game.
Finally, you should make sure you bring one Fire-type, Grass-type, or Water-type to deal with that Drillbur. Since Tepig are likely to already have evolved into Pignite by this point, which is a Fire/Fighting type, you’re probably better off bringing Charmeleon or Darumakka. Which one is up to you- Charmeleon is better when there’s little standing room, as in the middle of the map, but Darumakka packs more raw power on the important platforms that hold the banners, since it can run around to improve its power. Your water-type best options here are Piplup/Prinplup and Gyarados- Seel lacks the mobility and power you need here, and while Quagsire is fun, it’s inaccurate with its accuracy-reducing attack, so it’s more of an off-hand option. Gyarados, on the other hand, flies. Prinplup packs a move with a strange area of effect, though- its Bubblebeam skips a square, then hits the two after that- that kind of range is very useful in Illusio, where a lot of the map is made of empty space. On the off chance you’ve gotten one evolved, Swadloon will also do well against Drillbur, having the defenses and reach to handle it without issue.
If you’re feeling especially lucky and Go is part of your army, you can bring her along- she comes with a Sandile, whose accuracy-reducing attack can be useful against Drillbur’s Dig offense, and can also work decently with Bibarel, who is a passable wall and has the might to really abuse its Hyper Fang.
Once you’ve packed together an army around 1200 strength, it’s time to go after Kenshin in Illusio. This time, Oichi only warns you that Kenshin uses Psychic pokemon and not to let your guard down- she’s finally not telling you exactly which types to bring to the field anymore.
Your arrival in Illusio is greeted b y the appearance of Kenshin, who considers you carefully as an opponent. However, he judges you with but a look, telling you your abilities are not comparable to his. He then completely dismisses you, and one of his lieutenants backs up his pronouncement as final.... and you are turned away, sent back to Chrysalia (the only fortress you could strike at him from).
On return to the fortress, you wander somewhat glumly about in Chrysalia. Your state of mind is not helped by Oichi, who is lamenting that you aren’t achieving anything while Nobunaga is probably continuing to conquer more and more territory. Her sorrow is interrupted, though- Kenshin has sent his own forces to attack you!