Mob Assassin Builder is good choice for higher level players. If you are in urgent need of some RF online gold, welcome to place orders from us, we will deliver the gold at the lowest price in an instant. Therefore you can't take go there when you just start the game. ABs are found on solus, annacade and lures where ether level 135 requirement.
ABs element is terra which is vulnerable to FIRE elements (fire converted weapons/fire force skills) in RF. ABs will drop normal elems. Sometimes rare elems can get items like gli, rian and beam which used for armor, jet pack and weapon crafting. And you can get some normal and intense level 45-50 weapons and armors.
But ABs can only offer PT and/or EXP, when the player attacking has a higher than level(RF powerleveling) 40(or is leeching to a player w/ that level-PL). And an experienced high level player once told you that you can be safe from an ABs attack if you have 140+ dodg. At last ABs damage is between 9k-10k if you are not wearing any elems and armor. It based on your experience, ABs are the highest damager in settlements next to Pit bosses. AOE(area-of-effect = splash) damage ang mga AB, beware when you are low levels.