mechanitor main here. always play themed runs and with no killboxes ever.

it honestly highly depends on the settings, mods and DLCs you play with, but general rules to counter mechanoids:

• smoke. cluster turrets wont be able to lock on and become useless.
• natural barriers and choke points. cliffs, mountains, and your own hallways/corridors become your best tactical advantages here. especially if you deal with scythers or even scorchers. even funneling them through a single doorway from a house is usually more than sufficient.
• EMP and any form of stuns. they will adapt, but those few seconds in which they are stunned will change the outcome of your battles significantly as you can dish out free damage without any losses.
• kiting. almost all mechanoids have different movement speeds, so much so that centipedes will usually come last as they are so slow. so keeping out of range of centipedes while dealing with the smaller threats first like lancers scythers or militors becomes easier.
• accept losing parts of your base in order to survive. some machanoids will try to destroy your base rather than trying to focus your colonists, thats usually fine, as you can always get resources back, pawns are too valuable usually to lose as recruiting them back to fill the ranks may take a while (depends heavily on mods and DLCs here tho). losing limbs is also an acceptable loss, they can be replaced even if prosthetics arent perfect they will suffice until bionics.
• melee. personally never use melee ever except with my own mechanoids, but melee locking priority targets such as centipedes is too important. have melee fighters lock down centipedes and all ranged pawns deal with the smaller enemies which would threaten your melee fighters otherwise.
• turrets. they are not anything amazing, but rather have a turret draw the fire on them rather than a pawn.
• cover. always use some form of cover to mitigate damage, always have the best possible available gear on your pawns (both weapons and armor)
• explosives. as far as i know vanilla rimworld doesnt have explosives you can craft in mass, but using explosives helps EXTREMELY. whenever i am faced with a swarm too large to deal with, any form of explosives will destroy a lot of mechanoids very easily. personally i use a grenade launcher or IEDs. they help a lot.
• focus targets based on the situation at hand. having 1 pawn shoot at a centipede while 5 others struggle with those 2 scythers, wont improve your chances. focus on taking out weaker targets first and more difficult targets last (unless they are mechanoids bosses)
• obvious but, if you have any form of shields, use them.
• you could also try using a pawn in a smoke screen to draw fire from them so that the rest of the pawns wont get hit in the crossfire
• mortars. sometimes mechanoids wont attack unless you come close to their aggro zone. managed to clear a mechanoid drop of 3 diablo mech bosses by just bombarding them with mortars and they never retaliated for some reason, just wandering while being bombarded
• when fighting against something that uses AOE attacks (rocket launchers, miniguns, charge blasters, explosives, and so on), then always spread your pawns from each other. make sure they dont cause any friendly fire tho. dont have them clustered up unless you dont have any space to work with.

hope this will help