Useful strategies
--Sapper rush. Build lots of Sappers, and send them to kingdom come!
--Merchant Wagon rush. Take your hero to enemy base, place down your Marketplace and Merchant Wagon, steal gold, make sappers, bingo.
--Bombard. Your hero can summon Sappers on the spot; summon them behind enemy lines and let them do their job! Note: Kaboom! is not auto-cast upon summon, but its hotkey is A, so players should be able to quickly kaboom them.
--TRANSFORMA! Robometamorphosis is very useful; by turning a unit into a mechanical, it prevents most enemy spells, and it allows you to cast spells such as 'Makeshift Repairs' on your hero and Engineers. It can also be used in conjunction with Cloaking Device.
--Baneling Drop. Yes. Once you reach Tech level 3, you gain access to Zeppelins. Load Zeppelins with Sappers, and drop them off at enemy bases for a truly explosive result.
--Cloaking. Combine Cloaking Device with any of the above, and you get powerful stealth attacks.
-Makeshift defenses. Whilst you may think the goblins are weak in defense because they lack towers, you are wrong. Sappers may be used defensively, to block passages. Engineers can lay down Land Mines in choke points, and be loaded into Zeppelins and Shredders for safety. Cloaking can be used on structures, to perhaps buy time or hide expansions.
--Kaboom! can be cast at a location. So, if you see a shadowmelding noob night elf, Kaboom! her!
Strategies to counter:
--Scout. And respond correctly. Sappers fall victim easily to ranged units, and the hero is not too much to worry about if you construct a few defenses. Detection will prevent cloaking and let you see land mines. PoM for NE might finally be useful

.
--Turtle. If you can get three heroes, you will be able to cripple the Trade Prince. You should always set up towers, in case a Zeppelin wants to come by. You can also go Sc2 style and wall-in to prevent Sapper rushes.
--Aerial units. If you see that the Goblin player lacks Scorpions, go for it! Aside from the player using Zeppelins to Kamikaze!, they are helpless.
--Focus fire hero. However, a smart Goblin trade prince will always have a tp or staff of tele.
--Invisibility. The Goblins do not get detection (save sentry wards from summoned Temporary Vendors and Spying Device ability of Engineer) till tech level 3. Beware, however, that Sappers can still Kaboom! the ground and deal damage to invisible units.