Recommendations for your Avatar:

• It must be Female
• She must have Galeforce as one of her five primary skills (She must have been a Lv.15 Dark Flier class at least once)
• She must have Armthrift as one of her five primary skills (She must been a Lv.1 Mercenary class at least once)
• She must have Limit Breaker as one of her five primary skills (You need to buy Rogues & Redeemers 3 DLC to acquire this Skill!)
• She must have Tomefaire as one of her five primary skills (She must have been a Lv.15 Sage class at least once)
• She must have Ignis as one of her five primary skills (She must have been a Lv.15 Grand Master class at least once)
• Give her a Max Mt Nosferatu or Aversa's Night (rest of the forge points goes to Hit)
• If you don't have Limit Breaker skill yet, pair up with Chrom as a Great Lord and make sure the Avatar has A or S Rank Support with him to receive max benefits
• When all expectations are met, have your Avatar as a Sorcerer class

This whole set basically makes your (Female) Avatar almost impossible to kill. I'll give you a definitive explanation how this set works:

Armthrift is a godly skill where Luck*2 is the percentage where the weapon doesn't use up its uses. Say like you're using a Iron Sword at 40 uses left, but still has 40 uses left after combat. Having Max Luck is important. And if it's nowhere around 50 or higher, then you could pair up with Chrom until you have the Limit Breaker skill. Another useful ability which expands the Avatar's stat caps by 10 points. Allowing her to become even more powerful than usual. To the main point, in order to acquire Armthrift, she needs to be Lv.1 Mercenary at least once.

Galeforce is a female-exclusive skill, but can be passed onto a son if this is the latest skill (the fifth skill on your Avatar's current skills; the skill in the middle). This skill is extremely useful; when you successfully kill an enemy you get to move (and kill) again! To the main point once more, she needs to be Lv.15 Dark Flier at least once.

Tomefaire is also an incredible skill to have as it boosts Mag stat by 5 if you're using a Tome weapon. An extra "oomph" in power does not hurt at all. It simply makes combat even more solo'able! She needs to be Lv.15 Sage at least once.

Ignis is basically your Avatar's signature ability until you have Morgan (and another child if desired). Ignis is an excellent skill where it takes half of your current Str stat added to your Mag stat. Let's say your Avatar has 40 Str at the moment and Ignis skill is activated, the 20 Str will add to your Avatar's current Mag, say like 55 Mag currently and she'll have a total of 75 Mag. (And don't forget Tomefaire which will make it 80 Mag!) She has to be Lv.15 Grand Master at least once.

Limit Breaker is basically what you think it is; it is the ultimate game breaking skill which expands your character's stat caps by 10 points. Let's say your Avatar's Luck stat capped at 45. Now you can get 55 Luck as your new stat cap! That way, you won't need Chrom to pair up with your Avatar anymore! But you still need to during DLC maps, so... yeah, keep pairing up with him. This skill can only be acquired by completing Rogues and Redeemers 3 DLC Map at least once. You can clear it as many times to farm this skill. Additionally, you also need to buy this DLC map.

Rogues and Redeemers 3 is a very difficult map to clear due to the enemies having a variety of skills that will most likely screw you over. But...! With this set, your Avatar should have NO worries at all! As a Sorcerer class, with max Mt Aversa's Night and Chrom as the pairing partner, you can clear this map with only two units! Chrom alone will die regardless. So don't even try doing that. In addition, the Avatar should have ALL of her stats maxed, especially her Mag and Luck stat. Once you beat this map for the first time ever, now you have something you can truly break the game with! Of course, a Male Avatar can do this too with the same set, but will not have Galeforce is the main problem. But if you don't care about it too much, then go ahead.

This set is also useful for the last SpotPass map, "A Radiant Hero". Again, you can easily clear the map and get Priam with only two units with this set. Just keep using your Avatar as your Main sweeper. Also, Aversa's Night (and Nosferatu) heals your HP by half of the HP of your enemy has. For example, the enemy has 60 HP and you one shot it with 123 damage, you get 30 HP in return. Extremely useful, right? And guess what? Your Aversa's Night will NEVER BREAK! Amazing, right?

While it is time consuming, it is worth spending time to as this is the only way you can easily 2-man every map! Except for Apotheosis, which is technically a set Lunatic+ Mode. And there you have it! Your ultimate game breaking unit! And your Avatar outshines them all! I hope this helps those who are struggling with maps that needs clearing.