The Phantom is a challenging ship to learn, but very rewarding once you have mastered it. There are two upgrades that are, in my opinion, almost mandatory for both Echo and Whisper: Veteran Instincts and Advanced Cloaking Device.
The Advanced Cloaking Device will allow you to recloak after you have made an attack. This allows the ship to maintain that base of four agility dice. The only trick, is that you need to be shooting ahead of everyone else to keep this cloak up. That is where Veteran Instincts comes into play. Vet Instincts brings Whisper up to a Pilot Skill of 9, and Echo up to an 8 - both very good.
There are builds that obviously use several more upgrades than this, however they each have their own flavor. Some examples are the "Buzzsaw" builds that use Fire Control Systems and Gunner, and resiliant builds that use Rebel Captive and Sensor Jammer.
Phantoms hate to see Ion and Stress, both of these tactics mess up their delicate cloak-move-recloak dance across the battlefield. Phantoms also hate to see pilots that have an equal or higher pilot skill, as this likely means that they are being shot at prior to their cloak being reactivated. There are cards that are coming out with Scum and Villainy that will be additional nuisances to the Phantom, but that is a little ways off.
Overall the Phantom is a monster to deal with on the battlefield, and it can take over a game, but the ship is not unbeatable. I look forward to seeing what you are able to come up with!