I've been thinking about this some more, and this strategy won't work. First it relies on there being only two units on the board, but assuming that's the case there are more problems. The Locust moves 8/12, but on the turns it loses init and runs away it has to worry about being shot in the back, so say it reserves 2 MP for that, it means the Locust can only move 10 hexes away from the Manticore (Less if rolling maps isn't used) and if the Manticore closes to 8 hexes it can back off next turn and the Locust can't move into it's hex. Or on a turn the Manticore wins init it can abck off, meaning the Locust will have to close for two turns, taking fire the entire time.
You're assuming a totally bare, open field. If there's a patch of woods or hills larger than a hex or two within range, the LCT can run past them, make one turn, and the vehicle probably can't move far enough to get a clear line of sight. Even if the LCT runs straight away from the vehicle, the back-shots are going to be at 10+ to hit with long-range weapons (PPC, LRM), and a single facing change will prevent a back-shot and still maintain the +4 movement modifier, but allow a 4/6 vehicle to put any LLs or AC/10s into short range. The LCT can USUALLY survive a 10 point hit from a PPC without losing too much combat effectiveness, so the tank will most likely need to tag it several times to put it down. Staying at 8 hexes from the 'Mech means harder shots at 11+ to hit (4 base, +2 for medium range, +4 for the 'Mech's move, and +1 for the vehicle's cruise), and staying put means 10+ due to no movement penalty for the tank.
If the vehicle follows the 'Mech any closer, it still can't turn around and get far enough away from the LCT on the next turn to prevent the 'Mech from moving into the vehicle's hex if the tank loses initiative. If it doesn't follow, the LCT can use its move to loop around to set up a run from behind cover in case it wins initiative on the next turn. Basically, the tank needs to stay at least 7 hexes away from any usable cover.
Granted, the matchup is not a guaranteed win for the LCT by any means, but given reasonably suitable terrain, I'd put my money on a decent player winning with the Locust. On a totally level treeless plain, I'd bet on the tank, but all it takes is one lucky stomp attack by the 'Mech that immobilizes the vehicle, or one lucky hit that puts a leg actuator hit on the 'Mech and it's practically over, one way or the other. Initiative is going to be the deciding factor in a contest between a 20 ton 'Mech and a 60 ton vehicle that's roughly double the price.
Granted, it gets a lot more complicated on a busy field, with other stuff that can shoot at the combatants.