Lightning Blow — ruling 1

You may target any creature, not just one that doesn’t have first strike. If the targeted creature already has first strike as Lightning Blow resolves, Lightning Blow won’t affect it (since multiple instances of first strike are redundant), but you’ll still draw a card at the beginning of the next turn’s upkeep.

Lightning Blow — ruling 2

If the targeted creature doesn’t have first strike, granting it first strike after combat damage has been dealt in the first 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.

Lightning Blow : règles, fonctionnement et interactions (MTG)

Lightning Blow 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 gains first strike until end of turn. Draw a card at the beginning of the next turn's upkeep.

How it works : Lightning Blow

Original English text — Wizards of the Coast / Scryfall

#1October 1, 2009· Wizards of the Coast
You may target any creature, not just one that doesn’t have first strike. If the targeted creature already has first strike as Lightning Blow resolves, Lightning Blow won’t affect it (since multiple instances of first strike are redundant), but you’ll still draw a card at the beginning of the next turn’s upkeep.
No other card is named in this ruling.
#2October 1, 2009· Wizards of the Coast
If the targeted creature doesn’t have first strike, granting it first strike after combat damage has been dealt in the first 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.