Mine are probably woefully out of date or just wrong...
Group 1:
5/5 Killer Effects
5/5 Reward Recast
Group 2:
5/5 Beast Affinity
5/5 Beast Healer
Sometimes I'm letting the pet do all the work and sometimes I'm fighting alongside it. Even when I'm teamed up with people I'm often running off to solo... weird huh? Sadly, I rarely get to take advantage of monster correlation. I'm often using Yuly, Falcorr, Gerard, Nazuna or Lulush.
I'm not sure if I've offered any useful information, but I'd welcome any merit advice.
Its a rare/blatent occasion I would say someone is 'doing it wrong'
Feral Howl I only use for an extra proc or emergency situation.. which is not much.
I use Killer Instinct all the time now since they improved the duration & recast. It is a very elegant, very cool design approach that I think is very appropriate for bst. Its also very powerful yet has appropriate, built-in limitations. (just like charm did!)
I've always disliked Beast Affinity because it seems they make you pay an upkeep on your pets this way. It feels gimmicky and irritating... however from a use standpoint:
I would say its primary use is if you want a level 99 Sheep. Nazuna is a great pet, but I've never been huge fan of nazuna (I like lulush ALOT more).
The only other reason to have 5/5 Beast Affinity merits is if you want to use some of the 75-capped pets for something. You can get them up to level 90 w/ the capped merits & augmented monster gloves +2. The only pet I think is remotely worth it to have at 90 would be Carrie, because bubble curtain is 50% magic damage reduction. That is fantastic for magical tanking. However, if you are fighting something higher level, I'd rather use Gooey or element-appropriate pet and mdt axes.
My recommendation is, that unless you love Nazuna more than other merits, don't bother w/ 5/5 Beast Affinity.
Also, unless you really want a wide variety of pets (which I do) I wouldn't use any Beast Affinity Merits. If you are like me, and want all the post 75 pets for the lowest cost 2/5 makes sense, but more are not worthwhile unless you go to 5/5.
Beast Healer is a Solid very worthwhile choice, Its also not requisite in any of the fights I've been doing.
If you are all about pet only fights, or Pet Super-tanking:
Call Beast
Reward Recast
Beast Healer
These are your priorities to maximize pet tanking/survival.
Call Beast recast reduction is not nearly so critical now as it used to be since before the reward adjustment.
You used to need to ridethat Call Beast recast timer for most NM's. We played them w/ called pets like we did NM's w/ charmed pets.
However, when you can Cure 2.5k pet HP every 50~90s w/ reward alone (let alone throwing dawn mulsums in) There is alot of room to cure your pet instead of replace it. Then add Run Wild in if you really need help letting pet last until CB is back up.
This Means reward recast and call beast recast are really not necessary. They were necessary for some time but not now.
Tame recast is a joke.
If you are into killer effects (especially combining w/ Killer Instinct and Gausape) these are great and there is plenty of room to select what you like. These won't help your pet, but they can help master significantly.
There is also plenty or room to select Sic/Ready recast if that is what you like. This all depends on Charges for your favorite Ready moves, or the ready moves you want for a specific fight. This doesn't really matter for Ready move proc in dynamis. Most pets have a 1-charge move and if you are just doing a 1 charge move, you will not get pet tp fast enough that you won't be able to use it. (you would to attempt a 4th ready move less than a minute after the first to have any trouble here).
Sic/Ready recast is all about the 2 charge and 3 charge ready moves. Wild carrot is the one that jumps to my attention most. If you main-heal a fight using Lulush, there are times when the difference between 2 min and 1:40s can make a significant difference.
tldr:
Group 1:
TAME: Complete waste
All the rest choose to your playstyle.
Group 2:
Feral howl: 1/5 if you want an emergency use. anymore doesn't seem practical
Killer instinct: Big Bst Performance boost for master and party, but eats inventory to carry jugs for situations.
Beast Affinity: 5/5 only for nazuna; 2/5 for all 75+ save nazuna & clyvonne; No sense in using differen values.
Beast Healer: Very solid, always useful.