If you go with the dinos, take two.
Unless he sets you up to be counter-charged, charge him with the Stegadon at every opportunity. Even if you don't get into combat, even if you're 18" away, vanilla infantry has to make a Terror check and failing one of those can be a game-breaking thing. Set them up with an Iceshard Blizzard in the previous turn for bonus points. Carnosaurs & Skink Chiefs on Terradons with the Terrifying Mask (great for making Ld7 crew flee) are exceptionally good at this.
The Bastiladon is good for removing chaff and decent as a tandem partner for warmachine hunting. Unless you're unlucky, the many hits generated by the Beam of Chotec will lend you some 6's. The Bastiladon can also open up your magic phase.
Otherwise, TG ruin Empire infantry. Terradons are a great choice for redirection/warmachine hunting. If you proxy them as Rippers, they'll eat his knights (toad rage the knights, try giving the Rippers regen if possible).
Watch out for the Runefang, the Luminark, the Hurricanum (especially if the Runefang-bearer is nearby), the tank (pop it with Searing Doom/Spirit Leech), the Light council, Demigryphs, etc. Remember, the Stank has to move in a straight line (if I remember right) and benefits from a 360° charge arc.
I've had good success with both FoM & WD on a Slann, but much greater success with FoM. Fiery Convocation & Tempest are excellent against T3, and Drain Magic is great on a buff-reliant army such as Empire. Arcane Unforging will ruin his pretty items, and Hand of Glory is always nice. WBW your TG to a sweet spot, and it may be your opponent's end.
Frenzied Temple Guard (via BSB w/ a Skavenpelt Banner) are incredible btw. It sounds like you have no Core, however - you may want to proxy these as Saurus. If you go with Frenzied ones, take Spears - no parrying with Berserk Rage anyway.
An Oldblood cowboy w/ Crown of Command & other gear is nice for hurting demigryphs. Bring chaff! I like three units of skirmishers.
Sorry that this is all over the place - I'm going stream-of-thought here.
