Beastmaster is really a solo job, usually very self-reliant and not invited to join parties. This has become one of my fav jobs after I learned how to play it correctly.....and BST have a bad rep for hogging all the mobs, so be courteous of your fellow players and do not poach their mobs. You will find that your pet will be doing the bulk of the fighting for you.....reason why I mentioned that BST is a solo job, but please keep reading
If you use jug pets this makes it life really easy but if you are trying to charm mobs try not to charm above your level because you can lose the charmed beast in the middle of a fight. So I recommend Even Match or lower.
If you do lose charm in the middle of battle try to recharm the beast again....if your charm timer is waiting to reset you will be forced to kill both mobs. Typically solo leveling BSTs will chose to sub White Mage for a variety of beneficial reasons.
If you are using a charmed mob then the best trust(s) to use is one that enhances charm/charisma and/or reward. This being said the trusts with charm is Trust: Brygid which has constant Indi-CHR, others to include may be Trust: Kupofried for dedication (exp bonus), Trust: Kuyin Hathdenna for Indi-Precision (Physical Accuracy), Trust: Star Sibyl for Indi-Acumen (magic attack bonus), Trust: Moogle for Indi-Refresh (-1~3 mp depending on level), and Trust: Sakura for Indi-Regen (+1~6 depending on level).
If wanting to participate in fight then use a tank, a healer, nuker, and damage dealer Trust.
http://ffxiclopedia.wikia.com/wiki/Trust:_Brygid http://ffxiclopedia.wikia.com/wiki/Trust:_Kupofried http://ffxiclopedia.wikia.com/wiki/Trust:_Kuyin_Hathdenna http://ffxiclopedia.wikia.com/wiki/Trust:_Star_Sibyl http://ffxiclopedia.wikia.com/wiki/Trust:_Moogle http://ffxiclopedia.wikia.com/wiki/Trust:_Sakura