How is that even an argument ? This skill has pretty long cooldown, so using it -> burst, and then normally attacking, you'll have AT LEAST 80 BG before it's ready again (unless attacks are stopped or miss). But guess what, you don't have to use it for this missing whatever BG and use it later to solo-burst, providing party with 10 BG, or when she's at 100 BG, you can use it as damage buff, there are various options.
Gabriel has longer cooldowns than Diana does, and using Diana's skill to generate burst as mentioned above will give more BG to party than whatever can Gabriel's skill provide.
As for Saraswati: yes she's quite fast, that's not argument for saying that Diana is slow though.
Soloing with Solomon means Solomon doing all the job. Yes, you need at least as many characters as mission says. So if it says 3, you need at least soul and 2 kamihime.