increasing voltage using operational amplifier
I'm assuming that you are wanting to amplify a 6V ac signal into a 12V ac signal. Here's the problem; using a power op-amp from a 12V supply is not the problem but the amount of output voltage it can produce will be somewhat less than 12Vpeak to peak.

The output transistors will not be able to turn-on hard enough to produce 12Vp-p out. You might find a device that may achieve 10Vp-p however.

Moving on (and assuming you can produce 12Vp-p out of a power op-amp), the next problem is the current - you have calculated that the current will be \$\frac{25W}{12V}\$ and concluded that it will be about 2A - this is correct for DC output voltages but some way off the mark if you want an ac signal.

A 12Vp-p sinewave will have an RMS value of \$\frac{12}{2 . \sqrt{2}} = 4.24V_{RMS}\$

And if you want a power of 25W then the current will be about \$5.9A_{RMS}\$

You may want to rethink your requirements based on the above. Maybe you are more concerned about having a squarewave output and if so then the problem is finding a power op-amp that can produce 12Vp-p from a 12Vdc supply.

One option is to use a H bridge amplifier if the load you want to drive is suitable for this type of output connection. Another option is to use a transformer output to step up by (say) 25% to accomodate the power op-amp's inability to produce 12Vp-p.