Our Joule here is an amazing teamkiller. She has good utility, and she can jump away anytime from you and your teammates, saving herself. She also hides her UltimateTeamKiller untill she gets the opening, and she gets it, TRIPLE KILL!

The opening that you get to unleash your Beam is the most important in every teamfight. There are also many ways to get your opening:

1: Running Away: have the whole enemy team follow you as a line, which is easy as you rocket jump from them, then hide in a bush and then KAMEHAMEHA, and you'll get a triple kill if the three ticks where enought to kill them.

2: Stunns: have one of your teammates stun the squishest, and then release your inner robot energy on them. If your teammates don't have a stun, jump on the swuishiest, wait for his team to follow you, and then BANG, TRIPLE KILL!

3: Baiting: WHNE your teammates go low on health, they usually retreat. The enemy almost always chases IN A LINE, giving you the perfect opportunity to gain a triple kill and save your teammate at the same time.

Frostburn slows the target each time you thunders trike, but the biggest feature is that it also slows WHNE you land your ultimate, making it harder to escape from!
Shatterglass for pure Crystal damage....to destroy your opponent with your skills
Broken Myth to reduce their Crystal defence, and if they have none, to amplify your damage every second!
Sorrowblade to amplify your thunder strike by a great ammount! Also to increase the damage of your normal attacks
Crucible to increase health by a large ammount, and to block dangerous stuns and Ringo's ultimate

SITUATIONAL ITEMS
Metal Jacket to greatly reduce the ammount of damage each normal attack dealt by the enemy, also, it stacks with your perk!
Aegis to greatly reduce the ammount of Crystal damage dealt to you. It also increasing health and has the active of reflex block, so that we can also block an stun!
Aftershock since your Thunder Strike has low cooldown, this item is nice because it lets your basic
attacks deal a 15% of the targets full health as Crystal damage each time you use a skill, as well as 25% cooldown reduction.
Clockwork it gives you 50% cooldown reduction. Great for spamming your abilities and making your KAMEHAMEHA more often in teamfights.
Fountain of Renewal it heals you and your Intire team! It is one of the best items in teamfights for any character.
Eve of Harvest if your low on health and your ultimate got three ticks on the enemy but didn't kill them, this is a great item! It heals you whenever you use any type of Crystal damage and gives nice energy regent with good Crystal damage!

In Early game, focus on getting as much Crystal power as you could preparing for your ult. Crystal power also makes your jump sting much more. Don't engage on the enemy or gank them early game, since you are not at your best in it. Play defensevly untill you get your ultimate.

At Late game, you spark at teamfights. Follow the steps of finding an oppenning, and then use your ult. also, it is best to wait until your broken myth stacks up to 5, then unleashing your KAMEHAMEHA at the good oppenning. Use your jump to either run away or stun the whole enemy team, and use your thundestrike to keep your damage good. Thunder strike also works greatly for catching fleeing enemies when applied with a frostburn, since it gives good damage and slows them at the same time.

This is my first guide :) I tried to do everything I can to make it awesome. Thanks for viewing very much :)
And I hope this helped you climb up ;)