I went back and fought the terrible serpent again (I forgot what I was wearing before/the stats I had), now with the following:
Perception: 36 (39)
Reflexes: 39 (41)
Strength: 36
Will: 38 (41)
Hat: lucky cap
Weapon: heavy fishing pole
Offhand: Idoru nurse
Shirt: scaled vest
Pants: scaled pants
Gadget: TZR chipdeck
Power
2 Etheric Power
2 Evasion Power
10 Melee Power
2 Ranged Power
4 Stealth Power
Defense
5 Etheric Defense
7 Melee Defense
Attributes
+3 Perception
+2 Reflexes
+3 Will
Encounters
Slightly Increased Chance of Noncombat Encounters
Net
6 Memory
3 Processor
Other
2 Bonus Power when the Defender has no Technique
4 First Aid
1 Bonus to Fishing Checks
This time I was using ranged/fire hybrid deck, with no stealth tech whatsoever.
So, I guess it's probably fighting the serpent with scaled vest and scaled pants equipped.
I hope this helps.
edit: I tested a few more times without scaled vest or scaled pants equipped. Both are required - seems to be an "outfit bonus" to learn the technique. Side note, try ratty suit against rats? sigh, maybe not.