The first thing one must realize is that clearing the path for the Land Raider is the wrong strategy. It deals alright damage just by itself, but it also calls in reinforcements, which can be some pretty insane things (such as Dreadnoughts and Assault Terminators). Second, killing enemies isn't part of the winning conditions. If you get the Land Raider to where it needs to go, you win, no need to clear the map.
1) First thing you need to do is create an Enginseer if you don't already have one, and have it hug and accompany the Land Raider, potentially hiding away during harder sections. Your goal in this mission has now shifted; the Land Raider doesn't need protection, but you better protect that Enginseer.
2) Then, escort the Land Raider with the Enginseer and whatever measly forces you can scrounge up. Again, the Land Raider is not what you are protecting, it's the Enginseer, so feel free to pull your forces slightly back to not lose the Enginseer.
3) When possible (strongly suggest after every engagement), stop the Land Raider using the commander and get the Enginseer to heal it to full.
4) Continue onward and watch the Land Raider/SM reinforcements mince the enemies.
Repeat previous steps until you reach the end (you would repeat step 1 in case the Enginseer dies, at which point you park the Land Raider by an IG building and wait for your new Enginseer to teleport to it).