Goblin Lairs are the main way to rack up experience in this game. The difficulty's generally split into three stages:
Stage 1-6: Goblins have low attack power and HP. Clearing them quickly is pretty straightforward. Recommended Team: Both support characters can deal damage, and their skills are easy to upgrade. Tip: Hold off on using Sigrid's Ultimate skill until after Voresh uses his. Release Vidimir and Rowena's skills after Rava. On the second day, you can equip gloves with Wrath Wooden Claws. These will boost your damage output in Goblin Lair dungeons. Swap them out for better purple gear when you find it. The Goblin Lair throws 6 waves of minions at you. Use the Ultimate skills from two heroes to clear the first wave. Then, use the skills of your remaining three heroes to clear the second wave, and just keep repeating that pattern.
