How difficult will the level 49 team rocket leader challenge be?

While any answer really depends on what a Team Go Rocket leader's team is and that changes every month or so, searching YouTube with the phrase "Pokemon Go team rocket leader under 1500 CP" yields dozens of results. You could also try searching for "Pokemon Go Rocket leader lowest CP challenge" to find videos of players beating Team Go Rocket leaders with Pokemon well below the required CP.

It looks like it will require a couple matches to serve as recon, you will need to know exactly what team you are facing in order to keep the advantage while using low CP Pokemon, but you don't use up radars when you lose and the basic rules of fighting Team Go Rocket leaders still apply.

1. Make sure you have Pokemon that will resist the leader's fast moves. Because you have two shields, you can negate most of the damage from enemy charge moves but will have to tank fast move damage. Resistance will be key.
2. Start with a Pokemon that has a super effective fast move and a low energy cost charge move. This will allow you to burn the leader's shield quickly before they can use their charge attacks against you. Anything requiring less than 50 energy should work fine, I often use a Machamp with Counter/Rock Slide, or a Rhyperior with Mud Slap/Superpower.
3. Try to find common weaknesses between the first and second Pokemon, or the second and third Pokemon. This will allow you to maximize the advantage of extra energy after you knock out an opponent.
4. If your Pokemon have charge moves that lower their own stats, such as Wild Charge, Superpower, or Overheat, try to store multiple charge moves and fire them all together. You can then switch to a different Pokemon to minimize the time your Pokemon spend with a debuff. This is especially true for moves that lower your own defense, because most Pokemon will take too much damage after their stats are lowered.