Here is the full list of all 128 BATTLETECH achievements.
Accomplished a Career Mode score at Elite rank (>480,000 points)
Accomplished a Career Mode score at Legendary rank (>720,000 points)
Completed Career Mode with Iron Man enabled
Hired 100 MechWarriors
Hired 50 MechWarriors
Become Allied with the Taurian Concordat
Become Allied with House Kurita
Become Allied with the Magistracy of Canopus
Become Allied with House Steiner
Completed a Campaign with Iron Man enabled
Start a campaign with 'Mechs requiring 8 parts, then complete 10 'Mechs
Completed your first Career Mode campaign with any rank
Accomplished a Career Mode score at Veteran rank (>320,000 points)
Hired 25 MechWarriors
Won an online multiplayer match
Completed a contract with one mech remaining and no pilots killed
Retreated 10 times from a contract before any of your 'Mechs got destroyed
Ejected 20 times
Watched all of the credits
Won a 4v4 match with all of your 'Mechs being Locusts
Won a skirmish match at the War (25m) Lance Value Budget
Won 10 skirmish matches
Won a skirmish match at the Battle (20m) Lance Value Budget
Won a skirmish match with a Lance Value 5m less than the OpFor's
Won a skirmish match with more than 25m Lance Value on both sides
Won a skirmish match with a Lance Value 10m less than the OpFor's
Won a skirmish match with a Lance Value 15m less than the OpFor's
Lost to the AI one time
Won a skirmish match with an Atlas vs four AI Commandos
Won a skirmish match using fewer 'Mechs than the AI opponent
Won a skirmish match with three Commandos vs an AI Atlas
Fielded a lance of four King Crabs, won, and lost no 'Mechs
Fielded a lance of four UrbanMechs, won, and lost no 'Mechs
Your light 'Mech killed an assault 'Mech with a melee attack
Won a skirmish match at the Clash (15m) Lance Value Budget
Destroyed an UrbanMech with Flamers
Become Allied with House Davion
Retreated once from a contract before any of your 'Mechs got destroyed
Ejected 5 times
Destroyed 20 enemy 'Mechs with Death from Above
Become Allied with House Marik
Successfully completed a contract using only one 'Mech
Destroyed 50 enemy 'Mechs in melee
Removed all the limbs of an enemy 'Mech
Destroyed an enemy 'Mech with Death from Above using an UrbanMech
Have a 'Mech and its Pilot survive being knocked down, shutdown, and shot in the head
Acquired all 36 launch date 'Mechs
Destroyed 50 assault 'Mechs
Crowned a rightful ruler
Invaded a capital
Become Allied with House Liao
Purchased a new 'Mech chassis
Destroyed 50 enemy vehicles with melee
Completed 100 mercenary contracts
Destroyed 500 'Mechs
Saved up over 20 million C-bills
Maxed out a MechWarrior's Tactics stat
Liberated the world of Tyrlon
Refitted a 'Mech other than an Awesome equipped with three or more PPCs
Saved up over 10 million C-bills
Learned an important secret
Destroyed an enemy Orion with an AC/20 headshot
Destroyed 5 enemy 'Mechs with Death from Above
Liberated the world of Itrom
Destroyed an enemy light 'Mech with a Highlander Death from Above
Hired five MechWarriors
Maxed out a MechWarrior's Guts stat
Destroyed 50 heavy 'Mechs
Destroyed an enemy 'Mech's head with a Called Shot
Maxed out a MechWarrior's Gunnery stat
Completed 50 mercenary contracts
Destroyed 250 'Mechs
Refitted 50 'Mechs
Discovered ancient technology
Destroyed two 'Mechs in the same turn using Multi-Target
Saved up over 5 million C-bills
Destroyed an enemy 'Mech and your own 'Mech while using Death from Above
Captured a villain
Liberated the world of Smithon
Maxed out a MechWarrior's Piloting stat
Refitted a 'Mech with only Missile weapons
Refitted a 'Mech with only Support weapons
Refitted a 'Mech with only Ballistic weapons
Refitted a 'Mech with the maximum amount of armor
Negotiated for nothing but reputation in a completed contract
Liberated the world of Panzyr
Destroyed 50 medium 'Mechs
Refitted 25 'Mechs
Built 10 'Mechs by collecting/salvaging their parts
Started a war
Completed 25 mercenary contracts
Destroyed an enemy 'Mech with Flamers
Your pilot ignored the safety instructions and destroyed their 'Mech by overheating while firing weapons
Killed a Pilot without destroying their 'Mech
Completed 25 'Mech repair work orders
Destroyed 50 light 'Mechs
Missed a Called Shot
Completed 10 mercenary contracts
Destroyed 50 'Mechs
Removed both arms and one leg from an enemy 'Mech
Destroyed 5 enemy 'Mechs in melee
Recovered an ancient ship
Destroyed 5 enemy vehicles with melee
Negotiated for nothing but salvage in a completed contract
Completed your first mercenary contract
Refitted a 'Mech with only Energy weapons
Made a melee attack on an enemy 'Mech that was knocked prone
Suffered a terrible betrayal
Destroyed an enemy 'Mech with an Alpha Strike
Destroyed an enemy 'Mech's leg with a Called Shot
Removed both arms from an enemy 'Mech
Removed one arm from an enemy 'Mech
Established a history
Battled someone who has this achievement (and therefore has either battled HBS developer/BT tourney champ Chris Eck or battled someone who battled someone...who has)
Played 5 online multiplayer matches
Played 50 online multiplayer matches
Played 100 online multiplayer matches
Won 25 online matches
Won 50 online matches
Won 100 online matches
Lost 10 online matches
Won a match with three 'Mechs vs. an OpFor with four 'Mechs
Won 50 skirmish matches
Won 100 skirmish matches