I don't and will never recommend going through 3rd parties (the majority of which are known to be in for an easy buck and not really carefor your interest), as this is easily and much more reliably done by yourself.

A well written letter should do, in your case something like "As you know EC261 places the burden of proof on the airline to prove that the weather bad to the point to be "extraordinary and unable to foresee", something which you have not done and believe you will not be able to do as [all other flights operated normally that day/the METAR data for this day doesn't show anyhing unusual/etc]. Therefore I do not appreciate your attemps to claim you are not liable and urge you to either pay out due compensation, or provide said evidence, within the legal timeframe of 7 days, lest I launch a formal claim against you and a complaint to the relevant regulatory bodies"

METAR data is something you can research and decode fairly easily on the internet, and whilst the burden of proof is never on you, normally it helps makes your case.

Note that even if they can legally fob your article 7 compensation off due to truly bad weather (it does happen, though not as often as they wish), they can NEVER fob off your duty of care expenses (meals/hotels/taxis/ancillary costs such as nonrefundable bookings lost) as a result of the delay.