Post patch I'm still able to beat Dr. Lash though I kinda just was testing if I could even beat him(it) so I have extremely little insight on this, though I had pretty much up-to-date end game stuff anyways. From my experience my stats were:
Class: Smuggler
Stats: All maxed at 40
Weapon: Monofilament Saber (Helped me a bit with the extra 3 points in evade)
Gun: "Black Light" Ultraviolet Laser Rifle
Shield: Reaper Armaments Mark I Shield Generator
Armor: Weavemail Coat (This is by far NOT the most optimal choice of armor at all to use as you prolly want more heat resist from what I've read, but I had this on when I beat him anyways xD Goo Armor might be the best choice for this fight still since it has a high defense rating and very good thermal defenses.)
Accessory: A Light Jetpack (Can use a Salamander Longcoat to further boost heat resist, though personally with a Smuggler's natural evasiveness maybe 1-2, maybe three of his 6 shots even hit if I get hit)
Underwear: (I didn't have optimal ones but Thermal Undies might help a bit more with the heat resists)
Basically I would just Low Blow/Aimed shot, and pray that I don't miss a Low Blow. If I did, with my unoptimal setup, he might kill me on usually 2-3 attacks so I had at least some room for error. With Aim Shot while he's stunned with double shot each shot can hit for 60+ dmg x2 (90ish if crit) And Low Blow works fairly often on him from my experience. If you run low on energy or health, try to pop a stealth field and lick your wounds the next turn if you're really in dire straights, though I usually just prefer to be on the full attack since he can still likely one-shot ya if you're unlucky. I didn't have any success blinding him with flash grenade, but Low Blow is generally safer. I kinda RNGeesused to victory about 3 times but with a more optimal setup he's prolly easily farmable.
I haven't played Merc yet but their Take Cover and Rapid Fire along with Concentrate Fire kicking in during the duration of Take Cover can probably dish out the DPS for them, and using Second Wind if you get low on energy/health as an emergency. Tech Specialist might be hella trickier (Maybe near impossible) to beat him with since they're definitely more defensively oriented, which isn't a safe strat against Lash and Techies lack the means to be able to mitigate really heavy damage coming their way consistently (No natural evasion), as well as they don't really have a consistent high damage ability that doesn't drain their energy like crazy... (Looking at you Volley...)
Also might be helpful to pre-prepare for the fight by taking some dance lessons from Fyn in Tavros to boost your base evasion (Or damage, though evasion's usually more helpful IMO) and pop a Crystal Shard from Seifyn in Myrellion for the extra +4 defense. Maybe even stop in for a spot of tea with Aina in Tavros for a bit more max HP if you're really feeling frugal
