which f15 you've flight? A or C or E?
talking on hard setting flying f-15E loaded with MK-82 free fall bomb you have to flight on low speed nosing down your jet to the target while having the bomb ready to release press S key for air brake then adjust your sight bar line's dot on the target then hit enter and retract the air brake and don't forget to gain speed by 100% throttle then perform escaping from the target area for avoiding AAAs or anti air guns fire
note : free fall bombs doesn't gives 100% hit and 60% of free fall bombs drooping missed their targets , so try to use smart bombs such as gbu-10 or 12 leaser guided bombs or gbu-30 GPS coordinated bombs for more accurate hit and you can attack your primary target from long distance out of AA effective range so they won't shot you down instead of loading free fall bombs
free fall bombs give you the ability to attack your target from 1 or 2 km away for lets say good hit and might miss its target
thats about flying f15e on strike mission
and flying f-15c on a dogfight
when you switch on to the air to air using the gunsight in dogfight the sigh won't move , you have to use your keyboard keys ( arrows ) for keeping it on a air target
if you new to SF2 play on easy settings to gain some experience
note : gunsight will never lock on a target and keep tracking it you have to aim and track it your self and hit the fire key on the right time and the right position while dogfight for a kill shot