Is this sentence grammatically correct 'Be prowess as always'?

The noun 'prowess' describes a superior or great skill or ability, a talent, strength or courage; great progress achieved in any field. It also has a specific meaning of bravery, especially in fighting.

So, 'prowess' isn't something a person or thing can 'be'; it is something demonstrated, or achieved. The sentence 'be prowess as always' is grammatically incorrect. Similarly, the following sentences are grammatically incorrect:

'Be kindness, as always.'

'Be sympathy, ...'

'Be success, ...'

You might instead, correctly, say:

'Achieve prowess, as always.'

'Demonstrate prowess, ...'

'Show prowess, ...'

As well, you might qualify the type of prowess you're referring to, as in:

'The children of this family always achieve prowess in everything they study.'

'Those described as heroes are frequently people who've demonstrated prowess in battle or in meeting challenges considered unbeatable.'

'The prowess shown by students here is exceptional, as always.'

'Medals and titles are awarded by many countries to honor the prowess of their military heroes.'

In similar context and using a term other than prowess, you might urge a person or people to:

'Be brave, as always.'

'Be a hero, ...'

'Progress well, ...'

'Be highly successful, ...'

'Be courageous, ...'

Another view:

It is grammatically correct only if it means something like "Let prowess remain as it has been" or "Keep on personifying prowess." Neither of these alternatives makes sense, but at least they are grammatical.

If some other meaning was intended, then it is a sentence fragment, not a complete sentence.