Which sword? (Looks)

I personally just wouldn't use tempest blades because I can't see how it could be beneficial to your DPS and you get the attack bonus regardless. Not to mention it's just kind of obnoxious clutter.

Cast Tempest Blades and leave the swords out while you continue fighting. Once the boss is vulnerable, send the swords flying and summon the blades immediately after again and send the next set flying. It grants some pretty huge burst damage, especially when used from Final Form, and it's also handy for clearing out small mobs on platforms above the character if the player uses Grapple and then stops before hitting the top to send off the swords before falling.

I find it to be more of a lead in burst attack rather than something used for increasing DPS in between Gigas Wave spamming. Inferno Breath is the same way too. Use it before proceeding to the rest of the spam and try to keep enemies on top of the burning areas.