You have a common fault in your animation. You're copy-pasting the past frame and just whiping off pixels from it. Explosions are explodey, the shape changes drastically through it, and it should look violent and sudden. You need to draw a few keyframes completely original before you tween between them by copy-paste-altering. The explosion keyframes should be the following:
*impact - some people like to do the BLACK HOLE/WHITE HOLE fast frame switch at impact, it really gives it some oomph
-I'd go with notween or perhaps one very fast frame, then
*explosion stress frame
-one tween
*flame diffuse, gray debris dust perhaps
-two tweens
*flames out, debris and dust dissipating
-two tweens
for an end result of 12 frames or so. Played fast, this is a good end result to try for.