The Giant Mole is a regular boss you will face in Rookgaard, its spawn rate is around 3-5 minutes.
The Giant Mole can combo hard up to 300-400HP so prepare for a hard battle. He shoots some missiles and his close combat attack is deadly. Use your bow and keep distance and heal your self when possible to stay full HP most of the time. Only really high level players (Lvl 100+) with good melee weapon and skills can try to kill with melee attacks.
The room is big enough to run around between the stones while hitting the Giant Mole with distance.
You will need many pikes or potions to kill 1 Giant Mole, so make sure you bring enough supplies to kill it.
NOTE: After killing him, it's really rare but it can appear as a Diabolic version, so be extremely cautious if that happends.
Giant Mole loot may vary, but the most rare stuff they can drop are parts of the Terra Set (Terra Robe, Terra Legs..) and Patch Boots but expect many other end game items as well: Devil Helmet, Warrior Helmet…