Is it possible to beat Brock in the first battle with him?

Solution 1:

Yes, it is possible to beat him the first time you battle him. The battle ends, and the story continues as if you had lost (As per the story in the anime) which means you still need to beat him again.

I took him with:

• Butterfree: Lv 11
• Pikachu: Lv 12
• Pidgeotto: Lv 10
• Pidgey: Lv 10 (Didn't use)
• Ratatta: Lv 10
• Sandshrew: Lv 10 (Didn't use)

I managed to get a lucky Sleep Powder the first move - Onix missed his Rock Throw. I switched to Pidgeotto and Sand attacked him (maxed out accuracy loss). I then switched to Pikachu and Paralysed him with Thunderwave (He was awake by that stage), and started using Thundershock.

I got him to about half health when Onix got in a lucky crit Rock throw, and I switched to Ratatta and started with Tail Whip. Once his Defence was lowered enough I Quick Attacked him until he fainted.

Solution 2:

I caught a Psyduck in the lake to the right of victory road. It was lvl 21 and knew confusion, so it two hit KO'd onyx! Then he just acted like he beat me, and I was super confused.