The Secret Beach is located in the lower left corner of the Dark Portal (activate map to find Dark Portal). The arrow leading to the zone is invisible (click under your legs). It has the same normal monsters as The Beach (except Bonefish), with two boss monsters added (Water Spirit and Zombie Duck). 


Monster Type Level (Casual)
White Duck Normal 820
Dark Duck Normal 840
Mad Duck Normal 860
Mallard Normal 880
Dark Mallard Normal 900
Thunder Lizard Normal 920
Wind Lizard Normal 940
Water Lizard Normal 960
Blue Crab Normal 980
Red Crab Normal 1000
Black Crab Normal 1020
Water Spirit Boss 1100
Zombie Duck Boss 1200


It is recommended to use heal against the Zombie Duck.  

The Secret Beach is best farmed on casual difficulty due to the Zombie Duck's tenacity (Undead Boss), which means the normal Beach zone can provide comparable or better rewards in less time.

Clicking the purple right arrow allows you to draw around your character, granting you a much more customized avatar.