Tanks are strong, like all factories and can be countered easily, like all factories.
The basic challenge is anticipating the overall pattern of the enemy and exploiting it in the right moment. A wild mix of having intel and your available units at the right place. To have the knowledge is something completely different then actually applying it ingame.
Thanks are slow & expensive!
The average bot factory allows you to be all over the map, seeding destruction & distraction to the enemy, while gaining intel at the same time. There are various counters.
Naming a view, given there are more:
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Cloakbot. Have some Imps all over the place. Spam Reaver & Ronin. Don't suicide, press forward and repair units. Always good to raid with Glaives and be everywhere to keep them busy. Mono-spam Knights.
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Spider Mono-spam Recluse. Mono-spam Venom. Or get a mix of Venom & Redback. Mono-spam hermits. Stun with widow units out of place the get them with fleas
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Shieldbot Mono-spam Rogue. Mono-spam Dirtbag. Have some snitches all over the place. Or crush them slowly with Felon, Thug, Outlaw, Racketeer and 1-2 shields. You might play round to get the right mix
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Hover Scalpel + Mace. Like ratio 3-4 more scalpels then mace. Scalpel will destroy those heavy tanks, while mace keeps them at at distance. Or mono-spam Halberd push via some available pathing to the base or meaningful eco, to buy some time to go for something else or exploit this
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Jumpbot Keep tanks at distance with Placeholder, while pushing forward and kill everything with Moderator. Mono-spam Puppies. Get Jacks and rip everything apart. Be everywhere and raid stuff with a bunch of Pyros. Throw some tanks around with Jugglenaut. With the right timing Skuttle is very usefull against tanks as well
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Amphbot Slow them down with Limpets and clean it up with Duck & Scallop. Mono-spam Buoy. Later on Grizzly is also very effective. Use Lobster to throw either some tanks around, depending on the map in water or lava or some holes. Or throw your own units on tanks, with Limpets. Or throw tanks out of position and kill them
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Gunship Spam Gnats, stun tanks and kill them with Locust & Harpy. Keep them at distance and kill them with Nimbus. Mono-spam Harpy, slow them down and kill them. Revenant kill tanks in a reasonable way
Having written above, keep in mind the game pattern is a moving target and changing constantly. It's an adaption race. Depending on the setup a usual legit counter/unit composition might not a good idea. I recommend 1:1 to play around.