Thunder Strike — ruling 1

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.)

Thunder Strike — ruling 2

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.

Thunder Strike : règles, fonctionnement et interactions (MTG)

Thunder Strike is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

Target creature gets +2/+0 and gains first strike until end of turn. (It deals combat damage before creatures without first strike.)

How it works : Thunder Strike

Original English text — Wizards of the Coast / Scryfall

#1August 15, 2010· Wizards of the Coast
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.)
No other card is named in this ruling.
#2August 15, 2010· Wizards of the Coast
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.
No other card is named in this ruling.