Here is the item again:
Assaulted with green peas catapulted across the table from her brother's
spoon.
You correctly identified the fragment as a participle phrase. Assaulted, the past participle that begins the phrase, is the clue.
To fix this fragment, you could add a subject and verb, like this:
Mary Lou Ellen was assaulted
with green peas catapulted across the table from her brother's spoon.
Or you could use Punctuation Rule 3:
Main clause + , + participle phrase.
The correction would look like this:
Mary Lou Ellen ducked under the table,
assaulted with green peas catapulted across the
table from her brother's spoon.
Or you could use Punctuation Rule 4:
Participle phrase + , + main clause.
And the correction would look like this:
Assaulted with green peas catapulted across the
table from her brother's spoon, Mary Lou Ellen fired
back with a scoop of mashed potatoes.
Enjoy your crown!