Tounge Snatch - Aslong as you get the extend on the grab you could get anything else you want(lifesteal works good too).
Cloaking Skin - I only put use to this ability for the invisibility so again get w/e yet buy them after the other three rows are fully upgraded. Oh crap i forgot Additional damage on first attack uncloaked is sooo goood GET THAT.
Slash - This is his most important skill once it comes to being an assassin. Back stab is needed for this build!(Other upgrades I would highly reccomend are Attack speed, and higher damage)
Other upgrades - Health( SO you can be slightly tanky as you chase your enemies), Boots(Chase enemies down faster), and I usually go piggy bank for early double skills yet if you do not like piggy bank do not let this crappy build stop you.
(Notice: This is how I purchase so If you do not like it, go experiment differnt purchases yet do not buy upgrades from Cloaking Skin until you have fully upgraded the other three rows) At beggining game, I use piggy bank to grab the two skills. Farm up to start working on backstab. As soon as backstab is maxed either work on additional damage or attack speed. Then maxing the second grab the other skill not maxed. Then I would max surprise party mask for the addiotional introduction damage. Then I would grab max health, and boots to become tanky and catch foes fast. I would then get extension for Tounge Snatch to get a shit load of range. Lifesteal for tounge grab would be next, to get some decent health from grabs. Finally I would grab damge for tounges just for additional damage.
If losing or dying.
If you were to start losing or dying, maxing health would be usefull. Still max backstab which is where most of Leons damage comes in. If you are getting caught alot early grab the boot for extra speed.
Play Style and Tips
Alright we have talked about builds and purchase order now let us get into some strategy. Slash is Leons main damage, all of his damage comes from this skill making this important. Now some kids do not reccomend backstab, those kids are idiots. True you need to sneak behind enemies to proc additional damge but imagine full upgraded backstab with full upgraded attack damage, your targets will cry. Now Tounge Snacth is also important, true it does not do much damage yet it allows you to grave enemies into your slash(think blitzcrank from League). Cloaking Skin is a fun skill, remember kids once you are invisible turrents wont shoot you unless you stand cloaking infront of a turrent currently attacking droids. Cloaking skin allows Leon to escape as long as he is not taking poison damage or damage over time(becareful of gnaw). Cloaking skin also allows you to sneak up on low health foes. Also if you get surprise party mask you will do alost of damage on introduction.
So how about this scenario, You are invisble and a Coco is going back with only 2 bars of health. Initiate with a perfect Tounge Snacth and continue with slash until Coco dies(if you have to get hit by turrent for kill do it). Killing low health enemies in begin game allows Leon to get fed(WARNING: IF YOU GET SUPER FED YOU WILL HEAR THE EPIC ELECTRIC PIANO PLAY ALL GAME LONG or atleast until you die). If you are just real close to low health targets walk behind them and introduce with slash(backstab proc) and if target tries to flee grab them. Once you become fed just catch kids off guard(i.e. someone pushing turrent)with invis-backstab-then tounge if target flees.
Douchesbags that are harder to fight with Leon
Clunk - He has so much health, with atttack speed you can shred him easy yet It will be tough to escape his nukes.
Coco - She is pretty much anti Leon, he slow trail reveals leon if chasing and his ball of nuke knocks leon away, She is like Singed in league, BECAREFUL ONCE CHASING.
Gnaw - Poison thats just it. Fucking annoying poison.
Genji - "HI im GENJI. You chasing me let me just 5 second snare you :D?"
Lonestar - Knockback
Froggy G - Stuns and annoying tornado of doom
Thats about it so good luck learning, once you master this guy you will never want to play others again unlessyour that douche that gets bored of awesome awesomenauts.