No confirmed fix yet. Players suggest: Try doing a custom battle on Monster Island on Medium/Hard and destroying the stone towers (NOT the ones surrounding the lava pit)
Suggested fix (unconfirmed): Set the CPU difficulty to "Easy" and play aggressively. Use Godzilla 2000's sweeping tail attacks to stun enemies, or spam King Ghidorah's dive bomb.
Suggested fix (unconfirmed): Go to the online game menu and click "invite a friend" to play with up to four friends. Note that there is currently no 4-player matchmaking.
No confirmed fix yet. Players suggest: to be fair, once you get everybody there isnt much else to do besides grinding the gallery, enjoy the time while you have stuff to do in the game while it's there, I got a feeling it'll be a while until we get anything…
Suggested fix (unconfirmed): Change the AI difficulty setting to "easy" in the options menu. Alternatively, select Monster Island and repeatedly throw enemies into the lava while edge-guarding the volcano.