Zafreetis are slow and it may be difficult using them with Drull, since it will get too far ahead of the Zafreeti and possibly die. Zafreeti can be used with Gorgo but is not recommended because Gorgo already has extremely high health.
You can use any amount of Zafreetis on any champion, but they seem to work best with Gorgo because he is slow and has enough health that he can wait for the Zafreeti to catch up.
You shouldn't put a Zafreeti near or in a base that has Aerial Defense Towers unless you destroy them first, as they attack only airborne monsters (when you attack, always try to make more than one Zafreeti). You may consider using the Catapult to take out anti-air towers so that your Zafreeti can heal freely without being attacked.
Even if you put out the ADTs, your zafreeti's and attackers still have a chance to die or even get hurt. There are two ways to pair up zafreeti and another attacking monster. The first way is to group them up (Attacker, then champion, then zafreetis) I don't recommend this, because even though it makes the puuty rage effective, there is an extremely high chance at least one attacker will wander off, getting them killed if a zafreeti does not choose to follow. The second method is to fling 1 attacker (usualy a D.A.V.E) and a zafreeti (2 in case the monster needs more healing). I prefer this one since there is a lower chance of the attackers wandering off, as they will have to group together and then seperate to die. The only real downside is that putty rage and other buffs are less effective. If you don't mind this disadvantage, go with this method.
It is also not recommended to fling a Champion while Zafreeti is nursing a group of Ichis, and/or Crabatrons while destroying all the defense towers, because Zafreeti will be distracted. It will leave them, eventually killing them in the end, only healing the Champion (if the champion is gravely damaged).
Because Fomor is attracted to damaged monsters and the boost it provides is ranged, the combined use of a Zafreeti and a Fomor is great, as even if the Zafreeti heals the monsters it'll still fly over to them.
A common tactic is to use Project X, Crabatron and/or D.A.V.E. with Zafreeti. Project X has very high attack and low speed and Crabatron has high health and low speed. Because of their low speed, Zafreetis can keep up and heal the Crabatrons, therefore distracting defensive towers while Project X takes them out. When the defensive towers are destroyed, send in Brains and loot the rest of the yard. This tactic is very useful. When you are of a higher level, you can also use D.A.V.E.s and it will have a similar effect. However, Project X only attacks defensive towers, so when you send in your Brains they will be able to loot ALL of the Resource Harvesters, Silos, and the Town Hall, while D.A.V.E. only attacks whatever is closest.
Another good strategy to use with a Zafreeti is to have a Fomor, Zafreeti and Project X together as a fighting squad. The Fomor will turn the Project X into a PPX and boost the Zafreeti. The boosted Zafreeti will heal the Fomor and the Project X in return. This strategy is costly, but the loot taken will make the goo cost and time well worth it.
If you have a Drull or a Gorgo, you can hatch some Teratorns to protect the Zafreeti if the Sniper Towers or ADT's kill your Zafreeti while your champion goes too far away.
And finally, a very useful tactic which I call "Death from the Sky" is to hatch 6-40 Teratorns, 3-9 Zafreetis, and use a level 6 Fomor. Only Sniper, Tesla, and ADTs will be able to hit them, thus reducing the amount of damage your monsters take. When using this tactic, make sure your Teratorns and/or Zafreetis are upgraded to at least level 2. Your Fomor must be level 3 and above for this to work as well. NOTE: If possible, fling some Twigs in to take out the ADTs, as they can obliterate your attack. Also make sure you don't fling all your Zafreeti by your Fomor, as they will only or mainly focus on healing him only and not your Teratorns.
Good Champion Healing Tip: If you have the new world map, your monsters will return to the housings after a battle. Althoguh Damaged monsters cannot be flung in until they are healed. The champion however, can be. The strategy is called "Heal and Run." Choose a Wild Monster Yard, scroll to the very edge of the yard and fling in your Zafreetis. Then Fling in your champion BEHIND or UNDER your Zafreetis. The Champion should run toward the yard, allowing the Zafreeti to heal it, when te champion is out of range, just end the Attack and do it again.