When mech waves are useful:
Mech waves are the perfect wave to send when your opponent has a lot of fodder (workers, warriors, pikes, swords), archers to back up that fodder, and has traps and/or abatis. Also, mech waves work even when the opponent is online! For me, there are two great approaches to killing the army above. One involves some waves designed solely to kill the fodder followed by some scout bombs until it can be wiped out with 100k phracts.
The other way is in the picture. Both work very well, but the mech wave takes far less waves to wipe out the city and therefore is the fastest way to wipe out a city in case he is on or someone can log into the defender’s account to save him with a different attacking method. Mechs are very slow moving, however, and eat tons of food so there are downsides as well.
The reasons it works (in brief):
Mechanical waves (or “mech waves”) are attack waves that utilize ballista, catapults, and/or battering rams to hit the enemy. The best waves, however, utilize all three types of mechs as shown above. The main advantages of the troops lies in the following:
1. Ballista and catapults have the greatest range in the game (outside of city defenses) and therefore get to shoot at the defending troops at a safe distance.
2. Battering rams have tons of life (they start at 5000 life with 0 technologies while an archer starts at 250) and therefore rams take a long time to die.
This wave takes advantage primarily of the ballista and catapult range to take out all of the defending troops. The rainbow on the attacking wave helps slow down the defending troops from approaching the attacking mechs too quickly and allows the attackers to do the most damage before they take any hits.