Here is some general information about the Borrow-A-Hero mechanics and how to progress in difficult LS/DS/Cantina battles:

• It is more important to look for quality over quantity when it comes to adding allies. If you’re stuck on the later campaign missions it can be very helpful to borrow a maxed out character from one of your allies. If you don’t have any strong allies, you can look for some players to add on the official forums or the SWGOH subreddit.
• The most recent leader you used to complete a battle within an area (LS/DS/Cantina/Challenges) will be the leader that your allies can borrow from you for their respective areas. For example, if you complete a Light Side mission using Qui-Gon Jinn as your leader, your allies will be able to borrow your Qui-Gon the next time they attempt a Light Side battle. The system does have bugs as occasionally it will offer you a leader to borrow that your ally doesn’t even use or have.
• You can only borrow characters which were not a part of your team in your most recent battle within that area. For example, let’s say you complete a Dark Side battle with Sidious, Royal Guard, Geo Soldier, Phasma and IG-86. None of those 5 characters will be available to borrow as a leader the next time you go into a dark side battle. Since Sidious & Phasma are the most common dark side leaders, you will likely not see a lot of your allied leaders to borrow.
• Usually after borrowing a hero from an ally there is a 4 hour cooldown before you can borrow a hero from the same player again. However, if you complete a gear challenge (AGI, INT, STR Gear Challenges) then that cooldown is reset, allowing you to borrow a leader from the same user repeatedly until you run out of challenges. This is especially useful if you have few allies with strong heroes worth borrowing.
• In my opinion the best Dark Side heroes to borrow are Old Daka & Royal Guard and the best Light Side heroes to borrow are Barriss Offee, Ewok Elder & Old Ben. Also taunting characters are good to borrow as well, since your borrowed leader can die during a battle and as long as the rest of your team is alive at the end of the fight you will still 3-star the battle. Basically any character that helps your characters survive a round of attacks from the enemy. Unless of course you already have a tanky team composition that can survive a long time in which case you should be borrowing the highest damage dealer.
• You can 3-star a mission without a full squad of 5 players. This is beneficial if you have 2-3 really strong characters on your LS/DS teams but a big power gap between your next most powerful characters. You actually only need 1 character on your team to three-star a battle. I completed areas 1-3 on my alt account using only a single tanky leader and an OP maxed out character borrowed from my main account.

Here is a video I created of me doing this exact process to complete the Hard Area 7 Dark Side Battles: