Covert Ops depend on surgical strikes. This means, you either get them before they come in, or try to survive and fight them as much as possible, but it is impossible to simply “react” to a Covert Ops Attack.
First, a CO cannot kill ships, if the team watches each others backs. In fact, denying the CO to even have a chance to attack, is the best defense.
This includes providing vision and watching enemy movements. There is nothing more frustrating for a CO player if he cannot target singular targets, and has to stay behind cover. Long range weapons like lasers or gravi beamers should try to tickle cos even if they do little damage, just to get the pilot more exhausted.
Second, a CO most likely depends on his mobility to survive, and without Adaptive, they are Paper. AoE weapons are as deadly, as slowing.
Third a CO is never completely invisible.
Using a Recon might be a good idea for start, if you feel comfortable with it. Empire Recons have decent survivability and damage, but not the speed to go after the CO; but in an orbit fight, that does not matter. They can use Detection to make COs appear, but it’s not entirely fair in a 1v1 (not that it can’t be done, but if you are at that level you might not have problems flying a CO yourself anyway). Fed Recons on the other hand are almost as strong as a CO in a dogfight, but not very handy if you get dragged into the enemy team.
I would still look again at the Tackler. No, the slows are not useless, you just need exercise, when and how to slow. The CO is extremely hard to play, as you admit yourself, so of course, learning to fly CO in a dogfight fit (RF Blaster, Pulse, Shrapnel, Rockets) is also pretty handy. For me personally, Tackler is usually enough, except I meet one of the players who I know to be deadly (but they are deadly no matter what they play), in that case, I like to counter it with CO myself, just to make sure (or die trying), because its my most played role.
The ECM can be deadly to a CO, if it waits for its moment. If the CO is engaged in fighting an enemy, and tries to dodge a lot of fire, ECMs can do horrible things to it. So use your ECM layed back! a good CO hopes that you will fire your ECM too early.
Guards have slowing fields, which can be deadly aswell. For bigger clusters, Guards are especially effective with pulsars.
And of course, there is Mines and Facetorps, which COs usually do not like much, since most of them have no Emergency Barriers; however in T5 some do, so use with caution.
I would not give up on flying the CO myself either, since you can learn from flying how others counter you. Flying a CO needs constant exercise and is a drain on the pilot. You have the feeling of being a god on the battlefield, which can be squashed at the next corner by a player you did not expect. Using the paranoia or overconfidence of a CO pilot is important imho.
Most of CO play is trying to eliminate specific targets first, and getting the enemy to use their modules too early. Concentrating on survival if a CO is on your tail can be more important than killing it. Since every second a CO keeps blasting you, it exposes the ship. After all, once it’s dead, it’s dead.
Especially if you are the tackler, you want to be in the position to cloak quickly if attacked, or close to an ally where you suspect a CO will strike.
Currently, the Scatter Gun is another story, lets hope this gun, which is too powerful on COs, will disappear. Also, Orion nerf has been discussed often, since that is the only module which sometimes makes COs just too strong in T5, where the metagame is much less oriented on “tanking huge amounts of damage” with a single ship, like in T3.