When creating the Mega Mel for Alpha King Titan you must defeat Beta King Titan First to get the attachment to make a mega mek. A mega mek requires 4 meks to combine as one. You can use the base level 51 meks to create the Mega Mek. Once the fight is over you lose all 4 meks that you have used. So advised to use 4 level 51s as stated.
2 things: 1. Base level Mek are 51, unlocked at level 91, with blueprints for higher levels obtained through the normal way (titans or OSDs). 2. They starve without fuel, and need element to sustain themselves.