Rathalos is solid in CB, but if you have a new account with scarce resources, then lego books are probbably nonexistent.  It may be better to use epics & rares for now.  

Frozen Banshee (Poison) is an early-to-mid game CB monster and you may also get good mileage from her in dungeon progression.  

Scabrius (Poison & Dec Atk) & Cormac (Dec Def) are also pretty good for clan boss, but all the toons I have mentioned so far are squishy attack toons.  

You will need a Dec Atk toon and couple of strong supports to keep your attack toons alive long enough to do damage.  Rector, Lightsworn, Steelskull & Uugo are the best options you have for that right now.  Of the four, Rector & Uugo will be useful across many areas of the game for the the longest time and between the two of them, they offer Dec Atk, Dec Def, Veil (Dmg Mitigation), Healing, Cleanse & Revive.