They are not hard to get them into eating prepared foods to help suppliment their diet.
But....
Please consider what they go through which could happen just days before they get to your tank. Starting with capture in the wild, a boat ride, a car ride, held in tank, a car ride to the airport, dumped into whosesaler tanks, repacked, another plane ride, a car ride to the retailer, dumped into another tank. You buy it, repacked, a short car ride, dumped into your tank.
Now we expect this fish eat foreign foods?
To top it off this fish is very shy and a very slow moving especially to chase food. One of the best things is to house fish so that the food is going to sit for a while on the bottom of the tank, where the fish normally hangs out or travels over.
I have seen people using breader nets to hold the fish and train it to eat foods this way. This doesnt seem good, the fish is stressed and trapped in there and causes it not to want to eat or act normally. I think your handicapping the operation using this technique.
The best thing is a small tank with low flow and a few hiding spots. If the fish feels comfortable it will feed more readily.
I have used a long clear tube(undergravel filter tubing), as a laundry shoot of sorts to deliver the food right to the bottom of the main tank right near my mandarins favorite hiding spots. This works good If they are in a big tank with competition for food, they are just to slow.
The two I have now currently will eat food right out of the water colum and chase down food that flows by them. They will also eat pellets (Marine S).
Their favorite is blood worms, it also works out well since they sink easier than most frozen foods and it fits in their mouths better. I have been keeping them on and off for 18 years now BTW. Im making it sound alot easier than it is but im sure you get the idea.
If it sounds like to much get a scotter blenny (also a dragonette), they are much much easier in getting to eat prepared foods.