Heliolisk is an interesting Pokemon. It's as if Game Freak wanted the perfect Water Pokemon counter, and they bring up this. Immunity to Water type moves whilst boasting Electric STAB essentially makes it a Water type's worst nightmare. Adding insult to injury Starmie and Greninja are perhaps the only Water types that outspeed it in standard; courtesy of it's odd and signature 109 Speed in standard (it only shares this speed with Durant, which is not common) not even Keldeo can outpace it. Heliolisk can also be multi-weather dependent due to Solar Power which essentially gives a Life Orb boost except that it has the same boost as +1 Special Attack. It's also got sweet coverage moves it Surf, Grass Knot and Focus Blast, and a coveted Ghost immunity is also an edgy perk. It's notable flaws are it's extremely pitiful defense and a crippling Fighting and Ground weakness, two common physically based attacks. It's still a very good Pokemon, though, so if your team has problems with Water types Heliolisk is a pretty good pick.

Lightspeed
Heliolisk @ Choice Scarf
Ability: Dry Skin
EVs: 4 HP/252 Special Attack/252 Speed
Nature: Timid

Volt Switch
Thunderbolt/Thunder
Surf/Grass Knot
Hidden Power Ice/Focus Blast

A blistering 522 Speed makes Heliolisk an extremely potent revenge killer capable of outpacing Choice Scarf 108s such as Terrakion and Keldeo. Volt Switch works well in tandem with it's speed to gain early momentum. Thunderbolt is primary hard hitting STAB etc, but use Thunder on a Rain Team. Speaking of Rain use Surf on Rain teams to nail Ground types such as Gliscor and Hippowdon. Grass Knot helps against Gastrodon and maybe Quagsire that otherwise may be a problem. Hidden Power Ice dents things like Garchomp and Dragonite. Focus Blast doesn't help as much as HP Ice does but it's still an option. It notably hits Mega Lucario (although ExtremeSpeed and Vaccum Wave are a problem) and Magnezone,

Stallisk
Heliolisk @ Leftovers
Ability: Dry Skin
EVs: 252 HP/4 Special Attack/252 Speed
Nature: Timid

Substitute
Toxic
Parabolic Charge/Thunderbolt/Thunder
Hidden Power Ice/Surf/Protect

This only works in the rain where Heliolisk can stall the living daylights out of things with a Sub up. Sub blocks fatal hits/gives a shield etc. Toxic cripples most things, and helps with your Stall. Parabolic Charge may sound weird in competitive sets but extra recovery on a stall set isn't really bad. Thunderbolt is for consistent, strong STAB while Thunder is the strongest but will miss unless you're in rain (which you always should be). Hidden Power Ice rounds up pseudo BoltBeam coverage, although if things like Garchomp aren't your concern Surf is stronger. Protect helps with stall, again.