>>33298388>using Challenge Mode as an exampleGetting Challenge Mode from the get-go without Action Replay/PKHeX is pretty annoying, though. You'd need a friend who has already completed Black 2 specifically (White 2 doesn't work, that gives you Easy Mode) and you have to have the key transferred to you.
If you are the Black 2 player who completed the game, resetting your save won't let you keep the key for your replay run. You cannot get the Hard Mode key back from your friend, assuming you gave it to them, because players can only transfer keys they unlocked themselves. That means if your friend is a White 2 player, it will be impossible for you to ever get Challenge Mode.
Without Challenge Mode, B2W2 retains the same level of difficulty that BW had, with gym leaders capping at 3 Pokemon, and the Elite 4 using a team of 4.
By the way, Challenge Mode only makes major changes like that to important trainers. The other trainers are still just as easy, but have their levels ramped up a bit.