ok this is going to sound weird but have you tried flinging a tomb scorpion or 2 at it with magic assisted charges? the Killingblow and poison should give you a decent chance at taking it out (the rider that is). otherwise just try to get as much CR on it as you can things like that tend to fail when exposed to too much Resolution and with some luck you can break it and chase it DOWN!
other then that it's like a lot of those things out there (Steamtank/ Treeman/ Big DeamonS) try to ignore it and go for the rest of his army
Cheers CJ