Since tailsman races will tend to go heavily defensive on their ship designs to get full use of their never miss advantage, that is where you have to attack them. As LGM says you can go as large as you want on ship size, and take out the stealth/scattering armor and ECM. They are useless and it fress up more space for weapons. Tailsman is rather large, and he will have all those defensive techs likely so that makes the weapon differential even larger. Then dedicate a large portion of your fleet to shield depleting and weapon only weapons. The key is to get rid of his weapons as quickly as possible during the fight. A ship with a tailsman and no weapons can't hurt you.
Other than that you beat them the same way you beat anybody else. Hit his planets where he isn't, take them if possible, glass them if not. Interfere with his relations. Get the other races allied agastin him, or at least not trading with him. Explain to them how hard tailsmans are to beat and they may see it's in their advantage to attack before they get too big. Tailsman ships are expensive. The Tailsman and all the shields add up quick. Beat him economically.
If players that use Tailsman start getting teamed up on regularly and put out early they will have less of an incentive to use them.
Geoschmo