Aft phasers as well as seen in Paradise Lost, when it fights the Lakota. And I think a few of the broader battle sequences towards the end of the series. We don't see it often because the Defiant is so front loaded. The quads and quantum torpedoes are up front. The rear stuff is defensive "get away to make another pass" weaponry. As far as the question? Voyager would be shredded by either of the other ships in seconds. The Defiant was a purpose designed weapon. Heavily armored, insanely armed, dangerously overpowered, and specifically designed to blow whatever is in front of it straight to hell. The Sovereigns were post Borg designed Battleships. Abandoning much of the "science and exploration" philosophy of the already scarily militarily competent Galaxy class for greater offensive and defensive roles. The Intrepids were the then new mass produced general purpose fleet vessels. Light multi role cruisers designed for everything from emergency response, to border patrol and policing to science and light exploration. They were well armed, but not purpose built or focused warships. As for Defiant vs Sovereign. The Sovereign would win against a single defiant class, but would no it had been in a fight. Against 2 or 3 it would be overwhelmed.