Front-line trooper implies a heavy design with a mix of mobility, endurance and firepower.
Limited time implies "easier the better", which also implies cheaper units, or using as much existing, common tech as possible as opposed to something fancier. YMMV.
With these in mind, we'll skip any experimental tech, at least for this trooper unit.
70 ton heavies are reasonably economical with 280 Light Fusion Engine, and reasonably tough too. To compensate for mediocre speed, i added four jump jets. Since ballistics are heavy, Endo-Steel is basically necessary. Ten Double Heat Sinks are more than enough. Armament is simple: An Ultra AC/10 and LB-10X, each with two tons of ammo. 13 tons of Light Ferro-Fibrous gives max armor and each side torso has CASE. Finally, i threw a C3 Slave and Guardian ECM there (in case there's a C3 Master around), more ammo might've been better idea but since i didn't have CASE II for this, 20 rounds per gun should do just fine.
It is not flashy but it should be reasonably dependable trooper.
Perun PR-1N
Mass: 70 tons
Tech Base: Inner Sphere
Chassis Config: Biped
Rules Level: Tournament Legal
Era: Clan Invasion
Tech Rating/Era Availability: E/X-X-E-A
Production Year: 3075
Cost: 12 365 800 C-Bills
Battle Value: 1 620
Chassis: Unknown Endo-Steel
Power Plant: Unknown 280 Light Fusion Engine
Walking Speed: 43,2 km/h
Maximum Speed: 64,8 km/h
Jump Jets: Unknown
Jump Capacity: 120 meters
Armor: Unknown Light Ferro-Fibrous w/ CASE
Armament:
1 Ultra AC/10
1 LB 10-X AC
1 Guardian ECM Suite
1 C3 Computer (Slave)
Manufacturer: Unknown
Primary Factory: Unknown
Communications System: Unknown
Targeting and Tracking System: Unknown
================================================================================
Equipment Type Rating Mass
--------------------------------------------------------------------------------
Internal Structure: Endo-Steel 107 points 3,50
Internal Locations: 5 LT, 5 RT, 2 LL, 2 RL
Engine: Light Fusion Engine 280 12,00
Walking MP: 4
Running MP: 6
Jumping MP: 4 Standard
Jump Jet Locations: 2 LT, 2 RT 4,00
Heat Sinks: Double Heat Sink 10(20) 0,00
Gyro: Standard 3,00
Cockpit: Standard 3,00
Actuators: L: SH+UA R: SH+UA
Armor: Light Ferro-Fibrous AV - 217 13,00
Armor Locations: 3 LA, 4 RA
CASE Locations: 1 LT, 1 RT 1,00
Internal Armor
Structure Factor
Head 3 9
Center Torso 22 34
Center Torso (rear) 10
L/R Torso 15 22
L/R Torso (rear) 8
L/R Arm 11 22
L/R Leg 15 30
================================================================================
Equipment Location Heat Critical Mass
--------------------------------------------------------------------------------
LB 10-X AC RA 2 6 11,00
Ultra AC/10 LA 4 7 13,00
Guardian ECM Suite CT 0 2 1,50
C3 Computer (Slave) HD 0 1 1,00
@LB 10-X (Slug) (10) RT - 1 1,00
@LB 10-X (Cluster) (10) RT - 1 1,00
@Ultra AC/10 (20) LT - 2 2,00
Free Critical Slots: 0
BattleForce Statistics
MV S (+0) M (+2) L (+4) E (+6) Wt. Ov Armor: 7 Points: 16
4j 3 3 3 0 3 0 Structure: 4
Special Abilities: ECM, C3S, CASE, MHQ1, SRCH, ES, SEAL, SOA, FLK 1/1/1
EDIT Here's a C3 Master variant. It downgrades the armament to class-5 ACs to mount the C3 Master Computer. Light Ferro was swapped for standard plate, and the left-over weight was used for some LMGs and their ammo. Not the most effective thing but as a C3 Master, it should stay safe so the extra range makes this a sniper of sorts, even if it does have low damage output for its size.
Perun PR-1C
Mass: 70 tons
Tech Base: Inner Sphere
Chassis Config: Biped
Rules Level: Tournament Legal
Era: Clan Invasion
Tech Rating/Era Availability: E/X-X-E-A
Production Year: 3075
Cost: 13 963 800 C-Bills
Battle Value: 1 365
Chassis: Unknown Endo-Steel
Power Plant: Unknown 280 Light Fusion Engine
Walking Speed: 43,2 km/h
Maximum Speed: 64,8 km/h
Jump Jets: Unknown
Jump Capacity: 120 meters
Armor: Unknown Standard Armor w/ CASE
Armament:
1 Ultra AC/5
1 LB 5-X AC
4 Light Machine Guns
1 Guardian ECM Suite
1 C3 Computer (Master)
Manufacturer: Unknown
Primary Factory: Unknown
Communications System: Unknown
Targeting and Tracking System: Unknown
================================================================================
Equipment Type Rating Mass
--------------------------------------------------------------------------------
Internal Structure: Endo-Steel 107 points 3,50
Internal Locations: 4 LT, 3 LA, 3 RA, 2 LL, 2 RL
Engine: Light Fusion Engine 280 12,00
Walking MP: 4
Running MP: 6
Jumping MP: 4 Standard
Jump Jet Locations: 2 LT, 2 RT 4,00
Heat Sinks: Double Heat Sink 10(20) 0,00
Gyro: Standard 3,00
Cockpit: Standard 3,00
Actuators: L: SH+UA R: SH+UA
Armor: Standard Armor AV - 216 13,50
CASE Locations: 1 LT, 1 RT 1,00
Internal Armor
Structure Factor
Head 3 9
Center Torso 22 33
Center Torso (rear) 10
L/R Torso 15 22
L/R Torso (rear) 8
L/R Arm 11 22
L/R Leg 15 30
================================================================================
Equipment Location Heat Critical Mass
--------------------------------------------------------------------------------
LB 5-X AC RA 1 5 8,00
2 Light Machine Guns RA 0 2 1,00
Ultra AC/5 LA 1 5 9,00
2 Light Machine Guns LA 0 2 1,00
C3 Computer (Master) RT 0 5 5,00
Guardian ECM Suite CT 0 2 1,50
@LB 5-X (Slug) (20) RT - 1 1,00
@LB 5-X (Cluster) (20) RT - 1 1,00
@Ultra AC/5 (40) LT - 2 2,00
@LMG (1/2) (100) LT - 1 0,50
Free Critical Slots: 1
BattleForce Statistics
MV S (+0) M (+2) L (+4) E (+6) Wt. Ov Armor: 7 Points: 14
4j 2 2 2 0 3 0 Structure: 4
Special Abilities: ECM, C3M, TAG, CASE, MHQ5, SRCH, ES, SEAL, SOA