2 ULMFlyer
Regarding Su-25:
1. 7000 m is a "static ceiling", "dynamic ceiling" is much higher, at least 10 km is quite reachable.
2. To release an air-to-air P-60 missile there is no need to equalize the altitude with the target aircraft.
However, it looks quite unlikely that 777 could be completely destroyed by such a small missile as P-60. Unless it exploded directly in front of the cockpit making the crew inoperable instantly plus resulting in massive cabin depressurization....