"merciless act" is correct and usable in written English.
You can use it when you are talking about a cruel, unfeeling, or harsh action, usually done intentionally. For example, "The bully showed no remorse for their merciless act of pushing the child."
That merciless act destroyed Planck's will to live.
As Tristan, Register held all in reserve for his merciless Act 3 challenge, fearlessly moulding the role to suit his own mix of lyricism, angst and stamina.
"We have the power to ensure that this particularly merciless act of nature does not steal any more than it already has from children whose lives were so precarious to begin with".
Thus in talking of Boko Haram, he denounced the group's "merciless acts".
Mr. Keith said the bombings were "acts of merciless savagery which could only outline the sheer inhumanity of the perpetrators".
North Korea responded to the news with over-the-top anger, pledging "merciless counter-measure" for a "blatant act of terrorism and war".
A number of merciless self-portraits abound.
In June, North Korea denounced news of the film as "a wanton act of terror" that would inflict "merciless countermeasures" on the US unless it was banned.
During the summer, the North Korean regime warned that the film's release represented "an act of war" that would lead to "merciless" retaliation against the US.
During the summer, the North Korean regime warned that the film's release was "an act of war" that would lead to "merciless" retaliation.
During the summer, the North Korean regime warned that Sony's release of the film The Interview represented "an act of war" that would lead to "merciless" retaliation against the US.
