These verses speak about God's help in specific battles against armed enemies, not a religious duty to terrorize people. They establish that fear can fall upon aggressors in war; they do not make terrorism intrinsic to Islam.
The strongest form of the criticism
The critic's point is not silly. Quran 3:151, 8:12, and 33:26 do say that Allah cast fear or terror into the hearts of disbelievers. Quran 8:12 also mentions striking necks and fingertips. So the argument says: if terrorism means instilling terror, and the Quran praises God for doing this, then terror-instilling is not an abuse of Islam but part of its own scripture.
The moral worry is also clear: should a good God not soften hearts and reconcile people, rather than increase fear toward those who do not believe?
What the verses are actually describing
"We will cast terror into the hearts of those who disbelieve for what they have associated with Allah of which He had not sent down any