I am already playing a long term campaing as a Packmaster myself so I'm quite new to this but:
I would recommend to just have 2 animal companions:
1- Wolf: (as a damage dealer and obstructor) for the trip and some spells and feats that boost the bite attack
2- A dinosaur: (they got the better stats, as a tank) or in my case a flying unit (pterodactyl) to be more difficult to attack and kill since this will be your weakest one who would get the boom companion last.
I would play as the tank and dmg dealer of the group boosting full Cons and Strenght leaving Dex as the 3rd.
You should get 2 booms companions for both animals.
And here comes the tricky and super extra hard part:
I have read that in Pathfinder "Player Companion: Chronicle of the legends" you can have an alternate Capstone ability as hunter. Obviously you need to reach lvl 20 but that skill is awesome for the packmaster hunter since he receives an extra animal companion with level 16 as long as she had never multiclassed.
So my suggestion is: if you are playing in the long run, leave the lvl 18 bonus feat open to add to the new "capstone" companion all the bonus feats you got before, and also leave the last lvl 19 feat for another "boom companion" and at lvl 20 you would get the pay back of that inversion.
If you follow this at lvl 20 you will have: 1 companion at full level 20 (focus on dmg or tanking in here) + 2 lvl 14 companions. All of them with 5 team feats and the bonus skill (lvl1) ans outflank (level 2).
As for the bonus feats most of them are focused on AoO and is badass:
- Bonus skill: applied to survival or perception
- Outflank: compulsary as you will be tanking not shooting arrows.
- Paired opportunist: this is one of the best team feats! You get free AoO when your companion gets it (so better to have a spear in order to AoO when the enemy gets melee + you get to reach more enemies you wouldnt get if not in melee)
- Tandem trip: as a wolf is a good one (moreover if you take a second 20 lvl wolf since it applies to both) and produces lots of AoO!!!
- Disarm Partner: is a super nice one too since every companion will get to disarm the enemy if you fail (melee hits without weapon also generates AoO!!)
And you get one feat more:
- Precise strike: gives you +1d6 dmg for flanking not bad with "Blades Above and Below" since you dont need to flank anymore. But you need to waste 2 feats. Or wait to have 3 pets and flank naturally in a cross position.
- Broken Wing Gambit: my preference since you get a -2 to get hit but you generate AoO to your companions if they attack you and you get the fre AoO too since Paired opportunist.
- Extend the Bulwark: for defend companions
Remember to get the Combat Reflexes feat for you and each of your Animal companion.
As for spells, always focus in prebattle buffs or spells that work for minutes/hours.
I have found some that are quite awesome:
- Antropomorphic animal (3rd lvl that last flr at least 7hours) + Strong Jaw (2 sizes more for dmg) would get your friking wolf in a killing machine since he acquires 2 arms to wield weapons (no-profficient) and do weapon attack plus the bite one boosted! Carrying a full set of armour + weapons and boosting equipment will help!
- Shield companion (2n lvl spell that last more than 2hours) and let you take the dmg received, will prevent your companions to die.
Remember that you get also the pack focus so you can boost your companion a little bit and yourself. At lvl 20 you could boost free 2 companions + 1 to you/extra companion.