Let the cpu have "whisper of Revenge" then change the controller setting to "controller 1" (Data>Settings>Cpu name) After transforming into a Darkling control the CPU and your Hero to clash them.
If your turn just "attack" while CPU defend, if CPU's turn let him "strike" then you "counter" (dont use your character to "strike" because darkling have autocounter ability). Upon their defeat choose the rob command.
Silver Sword - Found in "lost forest north". When you're inside use 3 crystal or use multi crystal, then move 3 spaces - 1 down and 2 right. You will see that you're in a place where is no lane. You must carry the magic key to open the locked box. You can also find a warrior shield in the left locked box in the lost forest.
Punisher - Also known as Overlord Rico's weapon, this can be found at the locked box at the back of Overlord Rico. Notice that there are two locked boxes near Overlord Rico? Go right from there and there is a hidden red loot and move up 3 times to travel on the 3 red loot, then move left to land on the locked box.
Strongest Dokapon Sword and Dokapon Shield - These can be won at casino 500k, but the easiest way is to unlock the locked boxes at Overlord Rico's room. Left box Sword, Right Box Shield.
Aura Knuckle - Located in the sky castle, the only locked box at the top center. Angel Wings is necessary for this to reach Sky Castle. Obtain from defeating Rico Jr. Sometimes he drops a black diamond though. Sky Castle is located at the 3rd floor of Daunting Mountain.