Wrangler engy needs a buddy, simple as. Then he targets, the shield protects and his buddy repairs the gun. VERY difficult to take down then.
Mini sentry + wrangler is fail, the gunslinger is all about getting behind a team, throwing one down as a distraction and moving on to build a tele. It's incredibly useful and deadly if you drop it behind someone whilst they are focused on the cart/point. In all of these situations you need a pistol rather than the wrangler to defend yourself.