I have been playing Flux a lot lately and I've been playing around with some different builds with great success. This is the build / strat I have been using and after using it for a while, I think it is a much better way to play Flux.
First of all, I highly recommend having a look at this guide, the guy gives a great explanation and demonstration of how all of Flux's skills actually work. I personally don't agree a whole lot with the build that he provides but the information about the actual hero is great.
Also if you can't micro well, don't play Flux. He requires good micro in his stutter steps to get kills. IE: Surge > Auto attack > Run > Release > Auto Attack > Run > Auto Attack > Surge. You have to be able to Stutter Step to keep in range for your second Surge. If you can't do this learn to.
Early: Support / Harass
Mid: Support / Semi Carry
Late: Semi Carry
Team Fight Sinergies
Kraken is your #1 buddy / Enemy
Other Friends: Tempest, Magmus, Behemother, Plague Rider, Zephyr - basically any of those AoE bad asses you see in nearly every good line up.
NOT YOUR FRIENDS, if they are on the other team: Kraken
Offensive Kill lanes work well with flux, he has good potential to gather up some nice kills early game.
Combos well with: Panda, Drunken Master, Slither, Kraken, etc… Any heroes with stuns, slow or can benefit from positional advantages. If you open with a Surge and it is followed up by a stun, you should be able to catch them with a second Surge as they try to run.
Flux is versatile laner and can work well in almost any laning situation. He probably isn't ideal mid, but if you do end up in a situation where you are required to Mid, skip the blood chalice and get a bottle.
Early game items: Boots, Chalice, Str Bracer, Lifetube
Your aim is to finish off your NullStone and set up good teams fights and gangs. It is unlikely that you will be carrying the game at this point. So do your job and set up some good kills.
Items: Sustainer, Steam Boots, Blessed Orb, Wards if needed, finish your Null Stone.
Your aim once you finish your Null Stone, you need to get a Icebrand ASAP. Once you have your Icebrand you will start to really cripple their team, you will now pick up more kills and can set up more gangs. You should be Spamming Release on creep waves and farming at least 200gpm. You should be aiming to get your Frostwolf skull ASAP. Your buy order should be Icebrand > Blessed Orb > Glowstone > Recipe. Your priority when building items should always be based around extra Strength.
Why Nullstone? Well this is the main divergence in the build from the common builds. I find that Nullstone works perfectly with Flux, early game he is more or less support so you need some regen and you need something to help you farm. Once you have your Nullstone, if gives your enemy few options to escape if you catch them with a Surge.
Your other core option is to rush Icebrand, however I don't really feel like it adds much to your abilities Early / Mid game. Surge > Release > Ult if needed > Surge. Should kill them.
Portal Key, Behemoth's Heart, Frostfield Plate, Shrunken Head
Magnetic Surge - This is your bread and butter spell, early game in your lane if you can afford it you should almost be casting this on Cool Down. The damage is dependent on the amount of strength you have. Try to use it in situations where their creep wave is nearly gone and you still have some creeps up. This will provide a bit of chip-damage onto the enemy as they run back. Make sure you get some auto attacks on the target as they run away.
Polarity Swap - This skill is your second priority to level up. This increases your attack range (similar to the way Flint's passive works). ALSO it increases your cast range, which also increases the distance the enemy is dragged. I know right!
In a general sense this skill should always be Set to "Drag" (so the arrows point inside). The push feature is much more of a offensive support option, ie: your homie is getting bashed up and you wanna help him out. But even then if the target is isolated, you are probably better pulling them towards you, so you can turn on them with Release and hopefully get a kill out of it.
Release - This is your farm skill / slow. I tend not to level this up early because in general you won't have the mana to support your harass or get close enough to really utilize the skill. Once you have a Chalice / Life-tube or some kind of Regen you should be using this a lot, auto attack damage all of the creeps in a wave until they are very low and use it to try and kill all of them at once.
Discharge - This **** is awesome, if you can successfully use it in a team fight the combo should be something like Discharge > Release > Surge on any runners. You need to be careful of pulling in other AoE strength Damage heroes (Moraxxus, Kraken, Behemoth) because they can ruin your fun times.
This is an extremely versatile spell as it can be used offensively and defensively. If you get ganked and you push their whole team away, you stand a pretty decent chance of surviving.
This is a global spell, so be aware of your position in concern to team mates and enemies. Even though you might be on the other side of the map, you have the potential to seal any ganks that might be happening around the map or help team mates chase any runners down.
1. Magnetic Surge
6.Polarity Swap / Discharge - (if you have enough mana to use it or intend on attempting a kill in a short amount of itme
10.Polarity Swap / Discharge
11.Polarity Swap / Discharge