I decided to put this article together to fill in some extras after I saw Spira's guide on spirits that covers the rest of the information I would have wanted to add. I know this is a short article and at sometime it can probably be merged into someone else's strategy/playing guide for summoners.
This trick is one that I've found particularly useful. Generally speaking, Summoner has the potential to always get claim on any NM. It does require you to be quick on your toes, but due to the way it works, you have a minor advantage over many other jobs. Of course, if you don't have any "competition" this technique isn't really worth anything and when used in the wrong way, can get you quickly killed.
The basics of this technique are getting the monster's attention and then claiming the monster. Getting the monster's attention is where most of the "trick" in this trick comes from. If you attempt a bloodpact on a "standing" (not engaged or agroed) mob, the mob will ignore the avatar and attack you directly until the bloodpact resolves and does damage to the mob. This is where some of the danger comes in with indiscriminate use. If your avatar misses the mob, you will maintain agro. If you are doing things that cause more enmity, you will potentially get to a point where your avatar can't quickly pull agro away from you. The trick in this, is that when you attempt a bloodpact on a "standing" mob, there's a special enmity situation. The mob isn't claimed to you but you are on it's enmity list. As a result of being on it's enmity list, your avatar will "defend" you, causing it to move towards the mob. This will work as long as you are close enough to attempt the bloodpact. Your bloodpact does not need to be successful.
After attempting a bloodpact on a mob, the mob is moving towards you, and your avatar is moving towards the mob. The special enmity situation also means that if another character attempts a claim tactic (dia, provoke, flash, holy, ranged attack, etc.) any time after you attempt the bloodpact the mob will not be immediately claimed. If someone else tries a claim tactic, you're in a critical situation. Depending on the tactic used, the mob may start moving towards someone else. In order to seal your claim to the mob, you need to get some decent damage in and you need to get it in quickly. An avatar spell or physical bloodpact can do this for you. The bigger the other party's claim tactic, the bigger your bloodpact should be. Once you've landed your damage, most of the time you will have also obtained claim.
My personal method is to use ranged attacks (Meteorite for Carbuncle, Tier IV magic for the avatars, Nether Blast for Diabolos, and Dispelga, if conditions permit, for Fenrir). The benefit of the ranged attacks is that as the monster is moving towards you, it may end up in range of the first bloodpact, causing the claim to happen without the followup damage requirement.
Elementals Initial Casting
As Spira mentioned in the exceptional page on spirits, if an elemental is summoned, it's casting frequency is determined by a number of factors, and is reset by using assault or retreat.
It also, fairly logically, follows that using that factor of elementals you can always get an elemental's first action to be casting if you take the time to prepare and have the appropriate conditions. If you wait for the timer to elapse before engaging a mob and engage the mob yourself, you will trigger spellcasting from the elemental as it protects you. If you use assault, you will reset it's timer. By agroing the mob yourself, the elemental will get involved in the battle without being issued the assault command. If enough of the timer has been allowed to elapse, the first action will be a spell.
I typically only use this when farming. As much as the elementals are fun to use, this technique could be very dangerous on mobs that are too high of a level. If the elemental decides to use AM, the time between when it starts casting and when you die might be less than the time for it to finish casting.
Subbing thief has a few benefits, especially while farming. The little extra help from Treasure Hunter would be understated, of course.
Of probably the largest interest is the double to triple output of spirit taker when using sneak attack.
Typically, regardless of weather or other factors, this will allow me to use my avatar of choice when farming (even elementals) as the return from spirit taker is more than enough to keep my mp pool topped off, especially when coupled with the every-5-minute elemental siphon.
Subbing scholar has a number of benefits for regular play. You get access to reraise, cure spells, -na spells, drain, aspir, dispel, and sleep, all of which have their individual situational applications.
Possibly a more interesting use for subbing scholar is a simple effect method to get the conjurer's ring to work all the time, every time, regardless of race. It can be trying to get enough gear to lower the HP of a galka summoner enough to activate the conjurer's ring from swapping gear, for example. But, sublimation is guaranteed to lower hp exactly 25%. This means that all races have easy access to the affect of the conjurer's ring, without needing to rely on getting bashed.
Using Bloodpacts to Survive
When soloing or dealing with monsters that you're having trouble keeping agro on your avatar while fighting, there is a simple rule to follow that will have you, at the very least running away to fight another day rather than pushing up daisies.
The simple rule is to never try to squeeze a bloodpact out of your avatar right before it dies. Obviously, there will be exceptions during the course of your adventures where you will want to squeeze a magic bloodpact out before your avatar dies to make use of the TP it has built up. However, when soloing harder mobs, or in situations where hate isn't moving as it should in a group fight, squeezing a bloodpact out before your avatar dies can be the nail in your coffin. With summoners being as physically weak as they are, and summoner armor not being exactly geared toward mitigating damage, you'll need all the help you can get avoiding agro. A bloodpact is the surest way to transfer the focus of a monster's attention from you to your avatar. Curing, resting, and casting an avatar are all things that will gain you more enmity than you had prior. Using a physical bloodpact right after casting a new avatar will ensure that agro stays firmly on your avatar rather than on you.
Dealing with Agro
About the worst thing that can happen to the weakling summoner class is a surprise, especially when that surprise comes in the form of unexpected agro.
The simple, nearly obvious, but effect way to deal with unexpected agro is to quickly summon an elemental spirit. Elemental spirits cast in 1 second, compared to several seconds for any other creature at a summoner's disposal. When you cast your spirit, be sure to give it the opportunity to attack the mob. If you summon the spirit and run away immediately, the spirit may be unable to catch up to the mob that is pounding your face in.
Dealing with Multiple Agro / Links
Worse than getting agro from one mob, is getting agro from several mobs, or in possibly the most common case, when you get agro from a beastmaster or summoner mob and their pet.
Despite the possible panic multiple agro may put you in, do not immediately distress. A summoner can quickly handle being ambushed by up to 3 mobs at once. When you get agroed, your avatar will immediately attack one of the monsters if it isn't currently engaged. If, once your avatar is engaged, you use the retreat command, your avatar will attack another mob that is attacking you. Lastly, you can use the assault command to get the attention of another mob that is attacking you. At this point you won't have had to wait on any recast timers to get up to 3 mobs off your back. As soon as recast timers are out, you can dislodge 2 more every 10 seconds. You should also be looking for another exit soon if you don't expect your avatar to be able to defeat all the mobs before either dying (which can happen very quickly, obviously depending on the mobs' levels) or before you run out of mp to supply perpetuation costs with.
Note that if you are using shiva or leviathan, you can dislodge many many other mobs simultaneously using slowga or sleepga.