So had a game last night and played this set up

Jax with push the limit, royal guard tie, shield upgrade and targeting computer

Howlrunner with swarm tactics

Mauler mithel with expert handling

Academy pilots x 2

We are both new players so just playing around with different builds, but this one really did destroy his side . Iam pretty happy with the feel of it

Mithel is a beast when up close.

What can I do to make this even better? I know Jax could do with different upgrade cards but iam fairly limited to what I have .

Also what would be a good rebel build to counter it?

I know you said you were limited, but turning it into a Crackshot build might help. The problem is Crackshot is only available in the Hound's Tooth/Kihraxz expansions (1 per each). Something like this:

Carnor Jax (Push the Limit, Royal Guard Tie, Autothrusters, Stealth Device) - 34

Howlrunner (Crackshot) - 19

Black Squadron Pilot (Crackshot) - 15

Black Squadron Pilot (Crackshot) - 15

Black Squadron Pilot (Crackshot) - 15

Total - 98

View In (Yet Another) X-Wing Squad Builder

You could also sub out one of the Black Squadron Crackshots for a Draw Their Fire (to help protect Howlrunner).

In this list, they have 2 priorities -- Howlrunner and Carnor Jax. With the Draw Their Fire, Howlrunner becomes harder to take out, so it either takes the heat off of Jax longer, splits their fire between the two, or Howlrunner gives her buff to the Blacks longer. Of course, it's hard to drop a Crackshot, as the Blacks just scream for it.

If you are playing casual, proxy the cards you don't have just to see how you like them. Autothrusters on Jax are almost a must. Keep him in Range 1 of the majority of their ships and they become sitting ducks for your TIE swarm.

sub out stealth device for something actually useful (hull/shield) and it'll be golden

only soontir can really get away with Stealth due to turtling up with double focus

Wouldn't autothrusters on jax be a waste as he's going to be up close and personal a majority of the time?

Autothrusters helps against turrets. A ship's "firing arc" is the two red lines on the ship card extending toward the front. Even though a turret can fire 360 degrees, if you are out of that front "firing arc", the benefits of the autothruster still kick in.