Boros Battleshaper — ruling 1

You may choose zero targets for either or both instances of “up to one target creature.”

Boros Battleshaper — ruling 2

If you choose two targets for Boros Battleshaper’s ability, and only one of those targets is legal when the ability tries to resolve, only the legal target will be affected. If all targets are illegal when the ability tries to resolve, it won’t resolve and none of its effects will happen.

Boros Battleshaper — ruling 3

The controller of each creature chooses which player or planeswalker it attacks or which attacking creature it blocks, as appropriate.

Boros Battleshaper — ruling 4

A creature forced to attack does so only if it’s able to do so as attackers are declared. If, at that time, the creature is tapped, is affected by a spell or ability that says it can’t attack, or hasn’t been under that player’s control continuously since the turn began (and doesn’t have haste), then it doesn’t attack. If there’s a cost associated with having the creature attack, the creature’s controller isn’t forced to pay that cost, so it doesn’t attack in that case either.

Boros Battleshaper — ruling 5

A creature forced to block does so only if it’s able to do so as blockers are declared. If, at that time, the creature is tapped, is affected by a spell or ability that says it can’t block, or no creatures are attacking its controller or a planeswalker controlled by that player, then it doesn’t block. If there’s a cost associated with having the creature block, its controller isn’t forced to pay that cost, so it doesn’t block in that case either.

Boros Battleshaper : règles, fonctionnement et interactions (MTG)

Boros Battleshaper 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

At the beginning of each combat, up to one target creature attacks or blocks this combat if able and up to one target creature can't attack or block this combat.

How it works : Boros Battleshaper

Original English text — Wizards of the Coast / Scryfall

#1April 15, 2013· Wizards of the Coast
You may choose zero targets for either or both instances of “up to one target creature.”
No other card is named in this ruling.
#2April 15, 2013· Wizards of the Coast
If you choose two targets for Boros Battleshaper’s ability, and only one of those targets is legal when the ability tries to resolve, only the legal target will be affected. If all targets are illegal when the ability tries to resolve, it won’t resolve and none of its effects will happen.
No other card is named in this ruling.
#3April 15, 2013· Wizards of the Coast
The controller of each creature chooses which player or planeswalker it attacks or which attacking creature it blocks, as appropriate.
No other card is named in this ruling.
#4April 15, 2013· Wizards of the Coast
A creature forced to attack does so only if it’s able to do so as attackers are declared. If, at that time, the creature is tapped, is affected by a spell or ability that says it can’t attack, or hasn’t been under that player’s control continuously since the turn began (and doesn’t have haste), then it doesn’t attack. If there’s a cost associated with having the creature attack, the creature’s controller isn’t forced to pay that cost, so it doesn’t attack in that case either.
No other card is named in this ruling.
#5April 15, 2013· Wizards of the Coast
A creature forced to block does so only if it’s able to do so as blockers are declared. If, at that time, the creature is tapped, is affected by a spell or ability that says it can’t block, or no creatures are attacking its controller or a planeswalker controlled by that player, then it doesn’t block. If there’s a cost associated with having the creature block, its controller isn’t forced to pay that cost, so it doesn’t block in that case either.
No other card is named in this ruling.