Just to help out a touch, you can still make improvements to this build correct? As I recall you need Weaponry to meet the prerequisites of the Forge Gun and in turn this unlocks the weapon modules. Why not simply increase your damage with Complex Heavy Modules so that the forge gun gets more punch behind it. Also take my advice and get those prototype anti weapons. They are painful for the big boy marauder tanks. If there is other ways to improve the Forge Gun's damage potential and DPS (Damage Per Second) take that as well. The more damage you can do in a short time frame the better chances you and your group will have.

Another way to make sure you can keep that tank under constant pressure is to paint it as squad leader for destruction. This will give your squad encouragement to go after the damn thing even if they don't have miss because everyone loves extra points. Secondly this also means the tank will always be on your Tac Net HUD and thus you can always find it and intercept it so that it never knows a reprieve. But yes AV grenades (and grenades in general) sometimes just flat out vanish without detonating.

Third thing to consider is tanks have a blind spot directly behind them and to the left. The reason I say is most of the marauders I encounter have their turrets automatically pointing in the front and cannot spin it around to the back to fire on you except for the main one. The turret on top of the main one typically cannot turn to the left in this build and is at the mercy of the main turret's direction for the most part. So you can avoid being shot at by three turrets and instead only have two attacking you at once should they eventually spot you. They also cannot aim directly up so a dropship has a bigger advantage and can chase the tank down like this. Though do watch out for a nasty swarmer if this is the route taken.

This is just stuff I can think up off the top of my head.