The main strategy for Gear Raid 3: Stage 19+ revolves around the following:
Left Side: AOE non-magic damage dealer with long range such as: Razaak, Nyx, or Maul.
If the damage dealer is placed in the front tile, it gives them more time to DPS, but requires a healer.
If the damage dealer is placed in the back tile, they have less time to DPS (need better gear) but doesn’t need a healer.
Middle: Dolores and Idril are very important for this middle section, as well as another damage dealer to kill the boss and a healer to heal them.
Good single Target DPS: Silas, Calypso, Idril, Arrogance, Setram, etc.
Right Side: This side requires specific heroes to use the single tile on the right side.
The best heroes are: Hatssut, Alaura, or Vargus since they have invisibility.
Idril is especially important since she can help to kill enemies on the right side.
Lords: Aracha or Luneria are especially important for this dungeon because they give you the additional range to reach enemies sooner, giving them more time to kill the enemies.
Here are some video Guides: