From here on out, her abilities will be referred to by their number order.
Engage enemy gods in DARK stance with Hinder (2), then aim and hit them with your Decay (1). Dip closer in, while carefully positioning yourself behind the tank for a hit with your Repulse (3). Change to LIGHT stance (4) and quickly fire Restoration (1) at your opponent if they are weak, otherwise quickly turn and run while blasting Inspire (3) to heal you and increase your movement speed. Don't look back.
TL,DR: 2,1,3,4,1,3,4 Only use 2 when you need it.
Although you might not realize it now, by late game you will be reducing your opponent's magical protections, and then hitting them with your most powerful abilities, plus your magic power, plus your penetration. Even WITH defense items, your opponents will be struggling to sustain their lane.
Hel is a very unique god, seemingly harmless and secretly most deadly. She lures her opponents away from shelter with the guise of being weak and helpless, and then strikes with fatal speed and power.
Observe the figure below for team positioning. This will help you determine where to stand when you are playing as Hel:
-- IDEAL TEAM LAYOUT --
Hel (Behind ADC, behind Tank)
Hunter/Assassin (Behind Tank)
Warrior/Guardian (Front Line)
Hel, although not an artillery god, can still easily poke damage through her teammates and enemy minions for a powerful frontal attack. Her line attacks will penetrate the front line and AoE any nearby gods on detonation.
Powerful Sustain - Keep your allies on lane longer for better team plays and more XP.
Powerful Anti CC - Cleanse stuns, roots, and other CC debuffs from allies.
Godly Late Game - Hel snowballs from mid game up, dealing 600+ damage per ability.
Terrible Mana Reserves - Hel only gets a few spells off early game before heading home.
Terrible Health - Hel will die very quickly without the tank and friendly CC.
Very Short Range - Hel's abilities are very short range for a mage.
Also, make sure one of your allied players purchases Void Stone. This will compound your penetration and sharply increase your damage output throughout the game, even if the other team builds magic protections.
You will find that Hel performs especially poorly during early game, so be sure to avoid risky engagements. Your goal during early game is to sustain your teammates on lane. This will increase their XP consumption and tip early game team fights into your team's favor.
You will have your chance to fight when you buy your third/fourth item; by that time you should be able to do enough damage or healing to concern your opponents.