The basic idea sounds really cool, although this mission really rewards mobility. Dwarves and other slow units simply aren't fast enough to reach the edge with a token in 4 rounds, so they have a 50% chance to just lose. Definitely one to only play with someone who knows they can move fast enough to get out :)
Also are the percentages meant to be "units" or "points"? The former seems pretty easy to game by picking a few extremely chonky dudes and then a bunch of swarms you don't care about, and you'll be able to put down a bigger army than the defender from round 1.
(Oh and I think you can cheese it by picking up the objective and then boarding an APC and letting that move - but your opponent will probably have something to say about that)