Main Mission 16: A Rogue Mega Beedrill

When you begin Mission 15 and learn about three more Rogue Mega Pokemon, the Beedrill is listed as the first one you might wish to pursue. That's good advice.

From the lobby area of Hotel Z, head outside and around the side of the building and climb the ladder Taunie showed you earlier in the game. As you start along the rooftops, you'll find her waiting for you. Talk to her and she will teach you a new technique.

After you talk to Taunie, she teaches you the Roto-Glide technique. You can use it with your phone to give yourself a slight boost while you are falling through the air. You just press and hold the A button for a quick assist. The brief boost allows you to ascend to reach distant ledges, rather than simply falling to the ground.  You can use it to cross along the rooftops in Lumios City.

Use Roto-Glide to make your way across the rooftops.

The nearby gap is the perfect place to practice using your new Roto-Glide technique, since you can fall a short distance and them climb a ladder to try again. At least for me, the technique took a few attempts to get right. You jump out and then press and hold the A button, and you need to keep moving forward (but not before you gain enough elevation). I kept falling a bit short. Give yourself a few attempts until you can confidently cross the gap.

Taunie joins you when you reach the far ledge, and points out the nearby Rogue Mega Pokemon. She says you need to get over to it quick.

Head to the location of the Beedrill that's at risk of Rogue Mega Evolution.

As you look ahead, you can see that the Rogue Mega Beedrill is on a ledge you can't proceed to directly. There are roofs to the left and right, and you'll need to move along those to get close enough to approach, even while using your new Roto-Glide ability.

There appear to be multiple routes you might follow to reach the Beedrill, but I had trouble finding the right one. This guide outlines the route I found useful in the end. 

From where you stood with Taunie, look around the area. You should spot a ladder that leads to a higher roof with some Mega Shards positioned around the base of a ladder. Instead of worrying about those shards, drop off the edge of the roof onto the green rooftop below you, which has an angled ramp.

Past the ramp, there's a blue roof. Climb onto that blue roof and run along it to reach its far end. Ahead of that gap, climb up onto another green roof to your left, which has webbing extending toward the next roof.

Trot across that webbing bridge to the beige roof. Then cross that roof to reach a second web bridge, which you should also cross to the next blue roof. Next, proceed ahead to the green roof. Simply follow that green rooftop forward and to the right to find Taunie standing ahead of the Rogue Mega Beedrill.

Boss Battle: Rogue Mega Beedrill

Taunie joins you for this battle, and she brings along Meganium. The Beedrill also has allies, in the form of low-level Kakuna enemies. 

The Rogue Mega Beedrill is at level 41, so you'll want to have a Pokemon that is similarly powerful, and which can use ‘+’ moves to produce energy and fill its Mega Evolution meter. I went with Emboar and used Flame Wheel, which is my go-to move in situations like this one. The move happens to be super effective on Beedrill.

When the Beedrill is dazed, that's a great time to whip out your Mega Evolution, if your meter is filled. You can get in a few hits before your foe recovers, making short work of a chunk of its life meter. However, be ready for reprisal. The bee likes to use a series of thorn projectiles, and you will need to take evasive action so they don't hit you for a lot of damage and end the battle prematurely.

Keep your Pokemon's HP in good shape throughout the battle, and watch for indications of status ailments. The Beedrill is capable of inflicting poison, which will quickly drain HP if you don't tend to it. Also, remain aware of your position in relation to the Beedrill, so it can't surprise you by attacking directly. Provided you are careful, you should be able to emerge victorious.

Once you win the battle, you will receive Beedrillite. In a cutscene, a nearby pedestrian quizzes you on what just happened. Then the mission concludes.