Setessan Tactics — ruling 1

Each of the two creatures that fight deals damage equal to its power to the other. If either one of the creatures isn't on the battlefield when the ability that instructs them to fight resolves, no damage is dealt.

Setessan Tactics — ruling 2

You choose how many targets each spell with a strive ability has and what those targets are as you cast it. It's legal to cast such a spell with no targets, although this is rarely a good idea. You can't choose the same target more than once for a single strive spell.

Setessan Tactics — ruling 3

The mana cost and mana value of strive spells don't change no matter how many targets they have. Strive abilities affect only what you pay.

Setessan Tactics — ruling 4

If all of the spell's targets are illegal when the spell tries to resolve, it won't resolve and none of its effects will happen. If one or more of its targets are legal when it tries to resolve, the spell will resolve and affect only those legal targets. It will have no effect on any illegal targets.

Setessan Tactics — ruling 5

If such a spell is copied, and the effect that copies the spell allows a player to choose new targets for the copy, the number of targets can't be changed. The player may change any number of the targets, including all of them or none of them. If, for one of the targets, the player can't choose a new legal target, then it remains unchanged (even if the current target is illegal).

Setessan Tactics — ruling 6

If a spell or ability allows you to cast a strive spell without paying its mana cost, you must pay the additional costs for any targets beyond the first.

Setessan Tactics : règles, fonctionnement et interactions (MTG)

Setessan Tactics 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

Strive — This spell costs more to cast for each target beyond the first. Until end of turn, any number of target creatures each get +1/+1 and gain ": This creature fights another target creature."

How it works : Setessan Tactics

Original English text — Wizards of the Coast / Scryfall

#1April 26, 2014· Wizards of the Coast
Each of the two creatures that fight deals damage equal to its power to the other. If either one of the creatures isn't on the battlefield when the ability that instructs them to fight resolves, no damage is dealt.
No other card is named in this ruling.
#2April 26, 2014· Wizards of the Coast
You choose how many targets each spell with a strive ability has and what those targets are as you cast it. It's legal to cast such a spell with no targets, although this is rarely a good idea. You can't choose the same target more than once for a single strive spell.
No other card is named in this ruling.
#3April 26, 2014· Wizards of the Coast
The mana cost and mana value of strive spells don't change no matter how many targets they have. Strive abilities affect only what you pay.
No other card is named in this ruling.
#4April 26, 2014· Wizards of the Coast
If all of the spell's targets are illegal when the spell tries to resolve, it won't resolve and none of its effects will happen. If one or more of its targets are legal when it tries to resolve, the spell will resolve and affect only those legal targets. It will have no effect on any illegal targets.
No other card is named in this ruling.
#5April 26, 2014· Wizards of the Coast
If such a spell is copied, and the effect that copies the spell allows a player to choose new targets for the copy, the number of targets can't be changed. The player may change any number of the targets, including all of them or none of them. If, for one of the targets, the player can't choose a new legal target, then it remains unchanged (even if the current target is illegal).
No other card is named in this ruling.
#6April 26, 2014· Wizards of the Coast
If a spell or ability allows you to cast a strive spell without paying its mana cost, you must pay the additional costs for any targets beyond the first.
No other card is named in this ruling.