Ah, so the idea is that the Sylph Permanently grants the Dive ability to the creature then the creature needs to activate the dive?
Yes
This gives the card a problem. Cards can have only one active ability so when used on another creature its current ability would be lost.
My suggested course of action would be to have the ability dive the target doubling its current fixed attack. For a cost of 3 should be about right cause you would use it your best creatures.
Do you mean that the target creature doubles its attack strength for 1 turn?
Almost. Doubling and then Doubling again and again for each of your Sylphs could be overpowered so to contain it, it should only double the base strength and further dives from other Sylphs will only add the unboosted attack.
Ie 10/5 Dragon. Gets targeted by the Sylph becomes 20(10)/5. If the next Sylph targets the same dragon it will become 30(10)/5 a third Sylph Targets the same Dragon it will become 40(10)/5.
If we were to run the same situation doubling the current attack strength. The dragon will be 20/5 after the first, 40/5 after the second and 80/5 after the third.