I'm level 105 and use 2 maxed up supply trains for each attack...they significantly prolong the life of your troops. They are much better than battle tactics and if you use them carefully they usually survive battles, as they move behind your attacking front...most of the time! I usually deploy some basic troops and a canon or three first then the two supply trains, then a few more troops to hang around them. Without them it is much harder to take out a base over level 80. A useful technique is to centre your targeted rally on your supply carts if the health of your troops is looking feeble. That brings them back for a shot of energy and can also protect the supply trains if they are under attack. I also recommend a wall miner too...I often lose them...but they save a lot of time and lower the loss of other troops when you are faced with a tough wall.