Play Chapter 2: Ruins Of Crystal City - The Lost Vault, and proceed through the mission until you get the "Search for the leaders of the Insecticons" objective. Before you can fight the leaders, there will be three swarms of Insecticons you have to kill. Switch to Starscream, and clear out the waves of Insecticon enemies. After defeating the three swarms, walk over to the terminal and press X. Then, press B to back out of the terminal. This will save your progress. Pause the game, and select the "Restart From Last Checkpoint" option to start at the beginning of the swarm attacks again. Repeat this process as many times as desired. Note: You will also be completing challenges that give you Gear Boxes that contain x4, x5, and x6 XP multipliers to make the process faster. Additionally, equip any hack that will give you extra XP.
Play Chapter 7, and proceed through the mission until reaching the end. Hack the weapons terminal before entering the final turret section. Select "No Shield Regen" while hacking to get +300% experience points. Also, slot the x6 XP multiplier multiple times, if possible. Go to the Jazz prison area, and clear out the basic enemies. Once you are down below, go to the objective marker, and jump on the turret. After the enemies spawn, use the T.E.C.H. XP bonuses, and destroy each incoming enemy. Use each bonus as the previous one lifts. Then, either wait for Jazz to die or pause the game, and select the "Restart From Last Checkpoint" option before the end of this section to continue farming experience points. You will retain the previously earned experience points after restarting and can repeat the process as many times as desired.