Voltaik has below average health for a legendary meaning that is not that hard to KO. As well as this is just plain out speeding since if you can kill it before it attacks you then you have removed a huge threat from the battlefield.

Burn/Poison/Bleeding/Nightmares will all be good counter due to the fact that Voltaik will end up losing bits of health each time he attacks leading to an easy one shot if not KO'd due to the negative effects. Stun/Freeze moves are great ways to stop Voltaik in its tracks since it won't get any extra turns. It you find yourself having to vs a Voltaik take it out as fast as possible to minimize the damage done to your team. You can also get rid of a speed rune for a Strength/Stamina rune, and giving you team team speed runes to make up for the loss of speed.(I don't Recommend this however because if you other monsters get KO'd then Voltaik is a smaller threat).With all these good things going for him he has to have some sort of weakness right? Well yes he does. This will give you a higher chance of going before your opponent with your already high speed as well as a good amout of stamina to make sure you can attack as long as possible, as well as do more damage if you lose some stamina due to Nightmares or stamina leak letting letting you deal more damage per turn.

A set for Voltaik I highly Recommend is One stamina, speed, and strength rune. This means you need to find out how much damage you do on average to an opponent depending on your strength runes and Stamina runes. Note that a Voltaik with one level 5 strength rune does about 45,000 damage with one level 5 stamina rune. When using Voltaik I suggest that you focus your attacks on one opponent since this almost guarantees that one monster will die and the others will take small amounts of damage (Due to Static Shocks). Voltaik also has high speed like most thunder monsters making out speeding him hard without some pretty good runes. These moves spell BAD news for any water monsters you have, since Voltaik's AI tells it to focus its attacks on your water monsters.

Monster legend using three voltaik