I swear, if I weren't a Capellan right down to my shriveled black heart and rotten soul, I'd probably be a Leaguer if only because their designs make you think; and the Grand Titan is a fine example of that being a monster at short to mid range and possessing a token, but welcome, long range option. I'd probably be happier with ER lasers and heat sinks over Pulse, but relying on your lancemates to shoot you in and those lovely -2 TNs negate that issue. Though people tend to forget just how fast most League Assault 'Mechs are and how well they slot in with the average heavy 'Mech, allowing you to slot them in among the more common Orion, Archer or Warhammer. My personal favorite, though, is a Lance made up of a Grand Titan, Albatross, Awesome and either a BattleMaster, Cerberus or a second example of the first three 'Mechs. Gives you a nice, flexible Lance with short, mid and long range options that can keep up with and bully lighter non-cavalry formations or pick at and harass slower units.
As for that example of Optimus Prime smacking down an Atlas? That was, iirc, fought on Solaris VII in Ishiyama, which won them the attention of Earthwerks FWL who purchased both it and the Jackal schematics. Assuming regular pilots for them both and a dash of luck, I could see how the GT could bring down ol' skullface.