Here are the four corner maps and requirements to find them. As you can see, you have to do more work than just the storyline quest to find these maps (exception of A13 maybe)
A1 : L9 Challenge Amulet (Kill 7000 monsters)
A13: Kill 12 Apparitions Amulet
M1 : Kill 200,000 monsters Amulet
M13: K9 Challenge Amulet (Beat 100 waves)
Of these four, it is impossible to get the K9 challenge amulet to get the M13 map without premium (endurance mode). It may be also very hard to get the A1 map without premium, as you will need to anger a lot.