I think the best Mech Destroyer is the same it's always been, a U/AC-20 with speed and a small amount of armor. Sure, it'll get slaughtered in a real fight, but as a surprise it can be surprisingly decent.
Mech Destroyer
Mass: 45 tons
Tech Base: Inner Sphere
Motive Type: Tracked
Rules Level: Experimental Tech
Era: All Eras (non-canon)
Tech Rating/Era Availability: E/X-X-E
Production Year: 0
Cost: 1,948,863 C-Bills
Battle Value: 787
Power Plant: 270 Fuel Cell Engine
Cruise Speed: 64.8 km/h
Flanking Speed: 97.2 km/h
Armor: Heavy Ferro-Fibrous
Armament:
1 Ultra AC/20
1 Small Laser
Manufacturer:
Primary Factory:
Communications System:
Targeting and Tracking System:
================================================================================
Equipment Type Rating Mass
--------------------------------------------------------------------------------
Internal Structure: Standard 20 points 4.50
Engine: Fuel-Cell Engine 270 17.50
Cruise MP: 6
Flank MP: 9
Heat Sinks: Single Heat Sink 1 0.00
Control Equipment: 2.50
Lift Equipment: 0.00
Armor: Heavy Ferro-Fibrous AV - 79 4.00
Armor
Factor
Front 25
Left/Right 19/19
Rear 16
================================================================================
Equipment Location Heat Spaces Mass
--------------------------------------------------------------------------------
Ultra AC/20 FR 8 1 15.00
Small Laser FR 1 1 0.50
@Ultra AC/20 (5) BD - 0 1.00
BattleForce Statistics
MV S (+0) M (+2) L (+4) E (+6) Wt. Ov Armor: 3 Points: 8
6 1 1 0 0 2 1 Structure: 2
Special Abilities: EE
Just like real tank destroyers, it's better to leave out the turret so you can spend the extra weight on armor and have one less facing to protect, so even more armor.
I will say that yours is better for hovercraft capable terrain.
If you must have a turret, then the only option is to swap the U/AC-20 for a Gauss Rifle. Turret means you need range so you don't get pounded as much.
Mech Destroyer-G
Mass: 40 tons
Tech Base: Inner Sphere
Motive Type: Tracked
Rules Level: Experimental Tech
Era: All Eras (non-canon)
Tech Rating/Era Availability: E/X-X-E
Production Year: 0
Cost: 1,410,717 C-Bills
Battle Value: 883
Power Plant: 200 Fuel Cell Engine
Cruise Speed: 54.0 km/h
Flanking Speed: 86.4 km/h
Armor: Heavy Ferro-Fibrous
Armament:
1 Gauss Rifle
1 Small Laser
Manufacturer:
Primary Factory:
Communications System:
Targeting and Tracking System:
================================================================================
Equipment Type Rating Mass
--------------------------------------------------------------------------------
Internal Structure: Standard 20 points 4.00
Engine: Fuel-Cell Engine 200 10.50
Cruise MP: 5
Flank MP: 8
Heat Sinks: Single Heat Sink 1 0.00
Control Equipment: 2.00
Lift Equipment: 0.00
Turret: 1.50
Armor: Heavy Ferro-Fibrous AV - 89 4.50
Armor
Factor
Front 23
Left/Right 17/17
Turret 18
Rear 14
================================================================================
Equipment Location Heat Spaces Mass
--------------------------------------------------------------------------------
Small Laser FR 1 1 0.50
Gauss Rifle T 1 1 15.00
@Gauss Rifle (16) BD - 0 2.00
BattleForce Statistics
MV S (+0) M (+2) L (+4) E (+6) Wt. Ov Armor: 3 Points: 9
5 2 2 2 0 2 0 Structure: 2
Special Abilities: EE