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

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

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.

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.

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.