The creature avoids electrocuting itself by having some insulating material 'backing' the electric organ. So the electricity follows the path of least resistance (into the prey) rather than discharging back into the attacker.
If you want to incapacitate a big animal, target its heart or brain. Zapping the tail of a cow will just get you kicked. Jumping on it and zapping the chest area near the heart will kill it. There's an important safety reason for paramedics yelling "Clear!" when they use defibrilators: it would be rather embarrassing to re-start the patient's heart and accidentally stop someone else's heart at the same time!
Zapping the heart is definitely a killing thing, not a stunning one. Go for the brain for a stun. BUT read this article on electronarcosis in slaughterhouses (page 8) - the stunning is equivalent to a grand mal epileptic fit. It is not like getting zapped by Captain Kirk with his phaser on stun. Page 10 shows duration of the stun in various farm animals, and page 12 has the amps and voltages. Looks like 200V is the minimum acceptable in modern slaughterhouses.
As well as Michael Kjörling's suggestion of electric eels, you might want to look at Torpedo rays. Scroll down to the bottom third of the page to read what voltages they use and how they stun prey. That paper says they stuns them and then eat them alive. In the past I saw one which said the electricity can sometimes cause the prey to spasm so violently that it breaks its back, but I can't find that one.
If something is as big as 80 metres long, your creatures won't have an arm span long enough to encompass it's heart (I'm assuming they are human sized). They may have to form a team to take down a diplodocus or blue whale.