The Division players may be feeling a bit burnt out after the fairly challenging Shield Phoenix in August and somewhat tedious Talon in September. It should come as a relief that October's Shield, Rebel, is a bit easier to attain.
Technically, Survival was easy to finish with the recently finished Global Event Ambush, but Rebel should prove to be even less of a challenge and feel more along the lines of free Cypher Keys, Exotics and Classified items. For those unacquainted, Rebel will require players to reach wave 15 in Resistance.
Since The Division saw a surge of new players in recent months, it's should be explained that Resistance is basically the horde mode of The Division. Players go to West Side Piers, either solo or with a group, reach an entrance to one of the three arenas and queue up or just go in alone.
Reaching wave 15 is fairly tough on World Tier 5, but it also yields the best rewards. Therefore, if you feel like getting challenged while also having a good chance of Exotics and Classified sets, feel free to go with the hardest difficulty. On the other hand, if you're a new player that doesn't have a maxed out Striker's Battlegear, Reclaimer and such, you might want to just jump in and reach wave 15 the easiest way possible.
Luckily, there is no mention of having to reach it in World Tier 5, so players will be able to queue up in World Tier 1 and sweep everything until they complete the Shield. This could still prove challenging if players don't have any gear sets, but it's probably not the case for the most part, thanks to the Reactivation Caches Ubisoft and Massive Entertainment rolled out a few months back.
Another downside to this approach is that players will be limited green, blue and purple gear only as High End, Exotic and Classified will all be available only in higher tiers. Then again, the Shield Cache will grant Classified and Exotic gear, so you might as well grab it the easiest way possible.
At the time of writing on 10 October 2018, Rebel Shield was one day away from being deployed in-game, so you might check up on some Resistance guides or the wiki link in order to understand the basics.