Upgrade them, they're super powerful against any troop.
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4 max teslas in the core do work.
Archers and cannons can be great, but they have a fairly limited coverage.
However, I neglected my teslas, and I actually appreciate it now, because the loot demand on them is super low. Sure, level 5 to 6 is 3 million gold and takes 10 days. That means I essentially needed 300,000 / builder day to keep that builder busy for a really long time. 300k is nothing - I get that incidentally while raiding for DE/Elixer. So, by the time that builder comes free, I will have had plenty of time to get enough gold for the next level.
Wizard Towers from 6 to 7, however, is 766k/builder day, which is a bit harder to do.
Consider the incoming resource requirement when you have 5 builders, 2-3 of which will likely be working on defenses, and how best to keep all your builders busy.