The Teratorn is a very potent utility unit. The true power of it lies in its airborne ability and its speed. First of all, it is very useful in destroying unguarded anti-ground units and bypassing bunkers or traps. Use Teratorn as a vanguard to soften the opponent's defenses, then deploy your mine trippers such as Pokey or Bolt.
However, its damage is less than satisfactory. Its 300 firepower is only the same as the Fink's power, though once fully upgraded Teratorn can deal 700 damage. It is best to use him with his 'Ricochet' ability. With the Ricochet ability at level 3, each Teratorn can deal up to 1300 damage per shot. When it's in a group of 15-20 or more, together with level 3 Fomor, it damages a lot of buildings and can easily destroy defenses except for Aerial Defense Towers.
Beware! A level one ADT can kill 7 or 8 untrained Teratorns before being knocked down. Because of this, catapulting in Pebble Bombs and/or Twig Missles or sending Teratorns in after the ground troops knocked down the ADT can be more effective than just mass flinging Teratorns, but mind that trained Teratorns under level 6 Fomor's buff can last longer than 10 seconds when shot by a single level 1 ADT.
Tesla and Sniper Towers can also take down Teratorns, so it is recommended that they are supported by tank-like monsters on the ground such as Gorgo, D.A.V.E.s, Ichis or even Octo-oozes. But Teratorns are best paired with Fomor, as it can fly and provide up to 75% buff (under Level 6-3rd bonus level) which is relatively useful and more permanent than Putty Rage which lasts for a certain amount of time. A huge group can easily take down a medium-defended yard with low and medium-level defense towers other than ADT.
If the ADTs of your enemy are placed next to a wall outside his/her yard, fling a few Eye-ras in front of the ADT. At the right angle, the ADT will be damaged by the splash damage of the Eye-ra. Also remember to catapult at least 100,000 of both Twig Missles and Pebble Bombs to destroy the ADT faster. Fling in as many Teratorns as you can for maximum loot.
Before attacking players with Teratorns, you absolutely must lure out all the monsters from the enemy's Monster Bunkers. The only monsters that can effectively protect the base from aerial attacks are the Rocket D.A.V.E.s, so you will typically find them in the bunkers. They have high damage and can kill even a level 6 Teratorn in just two hits, so they are very dangerous. Eye-Ras can also kill aerial units, but they can be baited by Ichis (or Pokeys if you are saving housing space and goo) if the enemy isn't very smart and placed the bunkers on the outer perimeter of his base. So, if there are indeed D.A.V.E.s in the bunkers, lure them out with cheap monsters, and send 1-2 Eye-ras to kill them. However, if D.A.V.E.s don't have rockets, don't bother, since they won't be able to attack your monsters anyway. This is also the same with the Eye-ras. Fling in one monster to see if it has the "Airburst" ability. If they do, you must absolutely lure them out as their damage can kill a Teratorn straight away, and the splash damage it causes creates havoc. If they do not have Airburst, don't bother about them.
In the absence of Aerial Defense Towers, Teratorns can destroy yards and outposts in a short time with the "Ricochet" ability. A small group (6-8) of level 3-4 (with at least level 1 Ricochet ability) Teratorns combined with a level 6 Fomor can completely destroy Abunakki's base without any Zafreeti. High-level Kozu's base can be destroyed by more than 17 Teratorns together with a level 6 Fomor in a single attack. However, without the buff provided by Fomor, Teratorns are considered to be rather weak. Teratorn can also be paired with a Gorgo or a Drull if there are 2 or more Zafreetis healing it (not quite effective though). Without Zafreetis, Teratorns can be killed easily, especially if the person's yard you're attacking has powerful ADTs, a high level Fomor, or anti-air monsters in bunkers. Unless your Teratorns are higher than level 3 and the ADTs are only around level 1, don't bother, just keep on catapulting at them or if it is near a wall; fling in Eye-ras close to it and their splash damage will easily damage the ADT. A test showed that Teratorns under level 6 Fomor's buff can survive a level 4 Sniper Tower + L1 ADT for about 8 seconds.
You can bait Teratorns in the Wild Monster Baiter to test your aerial defenses as it is the only flying monster that can be baited.