Hypothetical Scenario

Let's say you're in a standard OU Battle. You and your opponent are both down to your final pokemon.
There's is a fighting type. Yours is a gyarados. Your gyarados is holding a gyaradosite, however, it has not mega evolved yet. Now...the question is, do you mega evolve your gyarados, strengthening it but giving it a type disadvantage? Or do you not, thus giving yourself the type advantage. I'm deliberately keeping the fighting type's species ambiguous because I know how many different scenarios that opens up.

assuming that your opponent knows it is the mega it is a 50/50, it is really up to you to decide that. However you must also take into consideration your opponents skill level too when making that decision. usually if they do not carry coverage that can hurt regular gyarados to badly you do not evolve