I follow a slightly different strategy. Lucia goes into my shore party as soon as she joins up, either because I recruited her early or because she signed up for "Convoy Strike". Then she stays there. She gets the same armour, weapons and medical supplies as other party officers, and she joins me in all battles on land or sea. So she soon learns the fine art of sticking your sword into an enemy while preventing him from doing it to you. By the end of the storyline, she's as fine a swordswoman as you could hope to meet, she's right there with me when I board the Dauphin Royal, and she's still there when I finally capture the ship. Sometimes I put her in command of it.
If you're trying to board a seventh frigate then you've engaged either a large patrol or a full fleet. I tend to avoid those unless all my party officers (including Lucia) have gold armour, good swords (at least swept-hilt rapiers) and good melee skill. Merchant ships are easier, provide good practice, and are also more profitable.