Orthus isn't a wimp now, he's just suicidal. The previous Orthus was a goddamn nightmare. Unassailable, he'd raze all of your improvements and then only attack when he had a near certain chance to win. To beat him you'd need overwhelming numbers of Bronze Axemen unless you caught him basically as he spawned. Now unless you have inadequate defenses he's no problem at all, just a free +1 Fire and Blitz promo...
Unfortunately on that spawn you have inadequate defenses, but you should still be able to take him by outsmarting the AI. Chances are he'll latch onto a unit and persue it across the entire length and bredth of Erebus trying to catch it. Proving once and for all, Orthus is no smarter than a bear.