Playing Jax well in a teamfight can make or break you. Knowing when to jump in, when to use your ultimate, when to pop your counterstrike, when to pop your gunblade active....all of those things require timing and knowing what your enemy team comp and playstyle up to that teamfight has been.
Let's break it down some, because a getting into a good position and knowing when to go in as Jax is CRUCIAL.
1:Enemy team has little to no CC
- Yummy, you will become a wrecking ball with this. Play this super aggressively, not being afraid to jump into the fray and initiate with your ultimate active+counter strike onto their AD or AP carry. A jax that can't be cc'd or kited is a very, very, scary thing. If you have initiation on your team, let them start the fight, then immediately Leap strike onto either the AD or AP carry, preferably the AD carry. As soon as you land, use your gunblade Active and immediately pop counter strike. They will try to run or flash away, but the Active on gunblade will still keep them in range of you by the time counter strike is up, if you played it right. Once counter strike goes off, pop your ultimate active, as you can usually wait longer to pop it against no cc teams because you won't get chained CC'd. If you get focus fired earlier though, pop your ultimate as soon as you leap strike.
Your burst damage is massive, and with the lack of CC, you'll be sticking onto the squishy like no tomorrow. They'll go down extremely quickly. After they are dead, you have plenty of options depending on how the fight has gone. If their AP carry is still up, get on top of him and start bashing his skull in, making sure to pop counter strike when it's up so he can't get away. If the AP carry is dead and gone, clean up duty is required. If you AD carry is getting molested by a tanky dps of some sort, pop your counter strike, leap strike onto the enemy, and immediately end the cs for the stun. This allows your AD carry a small window to put some space between him and the enemy, and if you get a TF/phage slow proc, it'll make it harder for the tanky dps to get to him. This will make him turn his attention towards you, or run. Either scenario is bad for him, as a late game Jax is a big no no to fight against without help. As always, if your Bot lane shut down their AD carry, then consider going after the AP carry instead. Whoever is the biggest threat to your team, shut them down.
2:Enemy Team has plenty CC/kite abilites
This is a bit trickier, and what seperates the good jax's, from the GREAT Jax's (badum tsh). A Jax that is CC'd to hell and back, won't do much in a teamfight. This is where I focus more on Burst damage with Jax, and not sustained damage, as you WILL be a high priority target to CC. Drop the Rageblade for an Early Sheen and Phage for maximum burst damage. After you finish Trinity, go either Abyssal or GA. This is where you have to time when you go into a teamfight correctly. DO NOT INITIATE, LET YOUR TEAM INITATE FOR YOU. Teams like this are where Ghost comes in really handy.
Let your team initiate, then WAIT for some CC to be blown. As soon as you see an Opening onto one of their squishies, Pop your ultimate, then Counterstrike, Then do an Empowered Leap strike combo onto them, You want to do as much burst damage as you can at once. As soon as you land onto their squishy, Immediately end the stun and pop ghost. Start whaling on them and wait for the inevitable peel/CC. As soon as you get it, Activate your gunblade onto the squishy, and start moving towards them. Ghost and the slow from GB will keep them in your sights. Leap strike onto them in neccessary as well, since it only has a 5 second cd with masteries and max rank. The advantage here is that if you built right, you will have already done MASSIVE damage to the enemy carry, before he could have even gotten away. Since you built more bursty, once they start throwing CC your way, it will be to little to late, as either A: you've killed the carry, or B: you've taken them basically out of the fight, having to blow flash over a wall and running for the hills. So basically the CC used on you was worthless. Meanwhile, your AD and AP carries are tearing them all a new one because they focused you when they shouldn't have.
An important thing to note here is that Taking a Carry out of the fight is almost as good as killing them. If they're extremely low and can't fight anymore, there is no use chasing them and abandoning your team. Let them run, Jump onto the AP carry, and make him sweat. If the AD carry comes back, he'll be extremely low, and a prime target for a W+Q burst.
3:The enemy team is heavily biased toward a single damage type (Heavy Physical or Heavy Magical)
The good thing about Jax, is he is very flexible when it comes to his builds. He's hard to counter build against, because His ultimate passive and Empower both deal magic damage, while Counter strike, Leap strike, and AA's do Physical. You counter his physical part, he'll build more AP and Burst. You counter his Magic, he'll build more physical and sustained.
He also has an advantage of being able to counter build others more effectively then most champions can, as his ultimate gives him bonus armor and MR based on how much bonus AD and AP he has. Enemy team really Physical heavy? build a BT after Gunblade, do more AA and Leap Strike damage, and get a ton of Bonus Armor from your ultimate. More magical? go Gunblade>Phage>Abyssal scepter or Maw>gunblade>Abyssal, and laugh as their burst does basically nothing to you, while your ultimate passive and Empowers hit like a truck. Build to counter your enemy. Most times though, you will want to go Hybrid because most teams are not like this.