Few things to approach here.
First off is that no legion games use remaining points of models to determine who wins (although it is used to break ties) so if you can play the objective better than him you will win, though that's a bit of a complicated solution that comes with experience.
Secondly, how is he engaged with you all game? there would be at least 1-2 turns at the start when you can hammer him with the saber tank. its guns + beam + armour piercing shells gives you critical 1 and impact 5 on a bucket of good dice on 2nd turn and crit 1 impact 2 on turn 1. on average with an aim you should deal 7.5 damage without accounting for stealing tokens from allies, almost crippling it.
Thirdly, get him to build the model. Most vehicles in this game have a hard time hiding for good reason so with it at full height it should be targetable from almost anywhere on the map (although it can also target you in return).
Republic are able to get a good load of activations into their lists and with him investing in 2 high value models you will be able to out activate him, forcing him to use the AT-ST on your terms.

Show the Quickbuilds!!

All card from recent article are ready. I plan to release update in the end of this week.
If you have ideas about proper cost of pilots/upgrades - you can propose your variants here.

If I wasn't at work, I'd tell you what's on those quick build cards myself. I'm guessing that they are still updating stuff on the site. Yesterday the updates trickled in over the course of a few hours. Someone will post the quick builds online soon though. If it doesn't get posted here in a few hours I'll post em myself once I can.

https://www.facebook.com/ProTechModels/photos/pcb.1821953371239302/1821952271239412/?type=3&theater&hc_location=ufi

Best rebel combination currently is giving Wedge proton torps (guaranteed crit going through) and something with coordinate to give him focus for double mod. So a mix of list 1 and 3. My idea would go around this
UT-60D U-wing - •Magva Yarro - 54
    •Magva Yarro - Cavern Angels Spotter (50)
        Tactical Officer (2)
        •Leia Organa (2)
        Pivot Wing (Open) (0)
BTL-A4 Y-wing - Gray Squadron Bomber - 41
    Gray Squadron Bomber - (31)
        Ion Cannon Turret (4)
        Veteran Turret Gunner (6)
BTL-A4 Y-wing - Gray Squadron Bomber - 41
    Gray Squadron Bomber - (31)
        Ion Cannon Turret (4)
        Veteran Turret Gunner (6)
T-65 X-wing - •Wedge Antilles - 64
    •Wedge Antilles - Red Two (52)
        Proton Torpedoes (12)
        Servomotor S-foils (Open) (0)
Total: 200/200

Agreed, max attack die rolled per turn is 8 with limited mods and def die is 3, 4 with Evaan's ability at max range of 1 from friendly ship. If I am across the table I am focusing ship by ship cuz I know you will fly a tight formation and 1 ship can only evade 2 hits, there will be melting.
My take on your concept
Lando Calrissian — Modified YT-1300 Light Freighter 80 Trick Shot 2 Veteran Turret Gunner 6 Jyn Erso 2 Nien Nunb 5 Ship Total: 95 Half Points: 48 Threshold: 7     Jake Farrell — RZ-1 A-Wing 36 Predator 2 Marksmanship 1 Ship Total: 39 Half Points: 20 Threshold: 2     Gavin Darklighter — E-Wing 61 Marksmanship 1 Fire-Control System 2 R4 Astromech 2 Ship Total: 66 Half Points: 33 Threshold: 3
A and E wings have more action econ. Being they are Jake and Gavin will give focus back as well as increase damage output with loose formations and evade die. My uncertainty with this comes to what EPT you use on the Wings and if Crackshot would be preferred.
Attack die roll per round - 12-13 with moar mods 
defense die roll per round- 7 with evades and arc

Idea is that lando can use perceptive co-pilot with jyn to get both a focus and evade in one action using second for whatever suits the situation. Evaan can spend her focus to bring Lando up to 2 evade with a re-roll for tanking. Not sure on ibtisam but had so many points left if AP5 was used instead.
C&C welcomed and highly appreciated
Lando Calrissian (80) 
    Hotshot Gunner (7)    
    Nien Nunb (5)    
    Perceptive Copilot (8)    
    Millennium Falcon (6)    
Ship total: (106)    
    
Evaan Verlaine (35)   
   Ion Cannon Turret (4)    
Ship total: (39)    
    
Ibtisam (50)      
   Jyn Erso (2)   
Ship total: (52)    
Total: 197  
                https://raithos.github.io/?f=Rebel Alliance&d=v5!s!43:-1,-1,86,52,54,-1,154,-1:;28:-1,138,-1,-1,-1,-1,-1:;80:-1,-1,40,-1,-1,-1:&sn=Unnamed Squadron&obs=