Thunder Strike

• WWizards of the Coast2010-08-15
  Official Ruling
  If a creature doesn’t have first strike, granting it first strike after combat damage has been dealt in the first-strike combat damage step won’t prevent it from dealing combat damage. It will still assign and deal its combat damage in the second combat damage step.
• WWizards of the Coast2010-08-15
  Official Ruling
  Thunder Strike can target any creature, even one that already has first strike or double strike. (If it affects such a creature, it will, in effect, just give it +2/+0.)

Rulings via Scryfall