Unfortunately, the Lakon Type 7 has the maneuverability and speed of a pregnant whale on land. Its shields are pretty bad for its size, almost regardless of shield generator. (A typical viper has better shields than a Type 7 with the biggest shield generator it can get, and the default shields is weaker than a sidewinder's A shield.). Its armaments are weak - 4 small hardpoints. Mine gets critical heat just from charging the FSD normally, let alone while using boost or in mass lock.
Basically, you can't fight in it. Your weapons are just for show. As for trading, while you have a bigger cargo hold than an Asp, you also can only use large landing pads and lose quite a bit of profits from that, and your maneuverability makes trade routes take longer. An asp is partly just as good a trading ship, and in additon, vastly better to fly, fight, and travel in - and it's what you 'upgrade' to the Type 7 from!
The Lakon type 7 has only one purpose - to make you appreciate other ships.
With that rant out of the way, what can you do about dumbfire missiles?
As I understand it, the PD turrets has not been fixed yet, so they won't help with the dumbfires. So, not being hit seems to be the best option. That is not easy in a Type 7, but a combination of boost, lateral thrust, heat sinks and/or chaff should give you a chance. Also, most importantly - get out of there.
Why do people use dumbfires? They act kind of like those helicopter rocket pods with tons of rockets that can be fired much like MG's, but unlike those rockets they're not firecrackers but reasonably full-damage missiles. Against large targets that can't or won't maneuver, they are fantastic. They do enough damage to destroy a large target, and they do it fast. They do have very limited ammunition, though.
Are they going to be nerfed? Yes, but don't expect it to be a case of OP->USELESS. Their role is to be good against whales, and most likely they will fulfill that role still.
Have they been nerfed allready? No. PDT's do not target them. This is, I believe, scheduled for the 'First week in february', and not the 1.1 patch, although things like that is seldom set in stone.