Oh, sorry, i didn´t read that it can´t be a flying monster.
Then, as the others said, the kadaai destroyer is a beast; BUT it has a "simple" countermeasure: a cheap hero, mounted on anything, with the dragonbane gem. 2+ ward against all the monster´s attacks (they are flaming), and can´t be thunderstomped = the über killing machine stopped (at least for 2/3 turns) with a model that probably cost less than a third.
Also, it has frenzy (and thus can be very easily redirected), and DOESN´T (read the rules carefully) have magical attacks, so a spirit host can hold it for the whole game.
If it´s not allowed, the HPA is still a very good monster. And the new kharybdiss of the druchiis (hidra is now rubbish) is quite good for it´s cost: only 160 pts for WS5, I4, A5, S7 stats.
Maybe the ancient stegadon?. With the "d3 wounds each impact hit" upgrade can give a lot of pain if you charge, and it´s 250 pts. Stubborn, ItP, cold blooded, make him hard to break (only Ld6, so remember to put him within the inspiring presence), and the giant blowpipes -2d6 poisoned shots EACH- come for free. I´ll give it a try.