Assail and assault share several definitions, including (1) to attack with or as if with violent blows, (2) to attack verbally, and (3) to beset. They are often used interchangeably, and they even share an origin—the Old French asilir (which in turn comes from Latin)—but there are subtle differences in usage: assail rarely refers to violence, instead referring to verbal attacks, while assault is usually harsher and often does refer to violence.
Somewhat confusingly, assailant, the noun corresponding to assail, often refers to someone who does violence. In other words, assailant often refers to someone who commits assault.
Examples
While a bevy of officials have assailed the proposed cap, the business community has come out to support it. [Scarsdale Patch]
Police are searching for a man who used a steel pipe to assault a Redan resident and smash the window of a car parked in the driveway. [Ballarat Courier]
In one fell swoop, Wal-Mart has addressed and disarmed three of the main brickbats that its opponents still use to assail the company. [Brand Channel]
The woman told authorities that Tawadros assaulted her several times in a storage room while her husband was picking up their two children. [LA Times]
It also led some oncologists to assail the ethics of the trial. [NY Times]
France’s government says it has signed a deal to sell assault warships to Russia. [Bloomberg Businessweek].