1. Floors:  Ancient Ruins becomes more difficult starting from floor 5. The Ancient Colossus will now have the following skills:  1) Rock Pound: Deals 180% P.DMG to 1 enemy, and decreases its SPD by 100.  2) Landslide: Deals 100% P.DMG to all enemies, and decreases the P.DEF and C.DEF of all targets by 30%.  3) Divide and Conquer: When Ancient Colossus drops below 70% HP, it will split into four small statues (they will merge into a Colossus again after three rounds).

Ancient Colossus' basic attacks now reduce the SPD of the character hit, thereby disrupting the player's action order. Take that into consideration if you plan on using certain skill combinations. Ancient Colossus' AoE skill reduces the P.DEF and C.DEF of all its targets, greatly increasing the damage it deals. It is worth noting that starting from floor 5, the small statues split from the Ancient Colossus will merge after back after three rounds. After the Ancient Colossus splits into smaller statues, defeat them as quickly as possible. 

2. Strategy:  1) As the clear conditions include having more than 30% HP remaining and losing less than 3 Saints, and as the Ancient Colossus' AoE skill reduces P.DEF and C.DEF, bringing a healer is recommended.  2) Since Ancient Colossus splits into small statues, it's recommended to include a Saint who deals powerful AoE damage in your lineup.

3. Recommended lineup: Kiki + Sextans Luna + Pegasus Seiya + Firebird Ikki + Unicorn Jabu + Eagle Marin

Your main damage dealers are Ikki and Jabu. With the help of Seiya's weakening skill, Ikki is able to quickly damage the Ancient Colossus until it splits. Jabu can then clear out the small statues with his AoE.

4. Cosmo 

It is recommended to use [Tenacity] as your Lunar Cosmo, since Ancient Colossus deals physical damage. This also gives you enough HP to survive. Your other Cosmo should be of as high a level as possible. If your support Saints are low level and have weak Cosmo, consider using Mandala or Hephaestus' Armor as their Legendary Cosmo to increase their survivability. You also need one Marsh Fairy on your team.  

5. Battle

Until you trigger the split, try to deal as much single-target damage as possible. When Colossus splits, use AoE to chip away at the statues' HP, and then finish them off with single-target skills. If the Ancient Colossus happens to cast its AoE, consider leaving some energy to your healer so that you can heal up.

6. Replacing Saints

1) Firebird Ikki can be replaced with other high single-target damage Saints, such as Scylla Io, Scorpio Milo, Capricorn Shura, etc.

2) Jabu can be replaced with other high AoE damage Saints, such as Phoenix Ikki, Cassiopeia Erda, Aquarius Camus, Leo Aiolia, etc.