Hoth Trooper with Atgar Cannon

Hoth Trooper with Atgar Cannon might be the weirdest piece in the game. It has the best hit points to cost ratio there is, but loses most of its utility after it takes a certain amount of damage. It's a Living piece that can be healed with Industrial Repair (as well as normal healing abilities). It's attack works like the Missiles ability, in addition to being a normal attack. It also has Speed 2 and Heavy Weapon, making it effectively even slower than Jabba the Hutt. Oh, and did I mention the very worst Defense score in the game?

Given all this, opinions are sort of sharply divided on the Atgar's effectiveness. No one (to my knowledge) thinks it's top tier by any stretch, but it actually can be used, and fairly effectively.
The first secret to using the thing well is to use it in multiples. It requires a fair amount of support in its squad, so you should probably maximize your synergy by having more than one piece that benefits from that support. Your first stop is the Rebel Captain, who can give the Atgar Twin Attack, thanks to the fact that it's called the "Hoth TROOPER with Atgar Cannon" and not some other variant of the name. Twin Attack makes these guys much scarier, because twice the splash and twice the chance of a lucky critical on that 30 base damage can only help you.

The other completely indispensable support is Admiral Ackbar (or a Rebel Officer, if Ackbar is unavailable). +4 to hit almost brings the Atgar to respectability, and vastly increases the chances of landing some of that juicy damage.

You should probably bring Princess Leia, Senator, who will let you move your Atgars 50% faster than they can go on their own. When you've got Speed 2, getting to move an extra two squares at the end your turn is a big deal. You might also consider Hoth Commander Leia, except that, since the Atgar stops being able to attack when it's reduced to 30 or fewer hit points, most opponents won't finish them off, so the resurrection Commander Effect helps less than usual. You know you can use the speed boost from Senator Leia, though, so that's my preference.

Finally, a piece or two with Industrial Repair is a good idea. Chewbacca, Rebel Hero; BoShek; or an Iktotchi Tech Specialist are all valid options. Keep those Atgars at 40 and above, and they can keep attacking. A sneaky thing to remember, however, is that even an Atgar at 10 hit points can still combine fire. Your opponent may forget about this, and sufficient combined fire can turn even a measly +3 Attack into a virtually sure hit, so remember combined fire if you choose to play an Atgar squad.

The bottom line is that the Atgar is not a bad piece, but it's definitely not easy to play. If you're looking for a challenge, try it out, but watch out for highly mobile opponents.

Overall rating in 100: 1.5 (tough to get sufficient support in here)
Overall rating in 200: 3.5 (if you can get them pointed at the main fight, they can be pretty effective backup)