I clear GO with 'newbie' account before and only use event eidolon. I oneshot GO with glass canon build too current GO i oneshot all mission using fire with event eidos and no flb weapon. With proper strategy GO is easy but most people usually going too offensive that make it takes much longer. Like in this GO for instance you can stall a lot in each wave except miniboss. When stalling you always have full burst, full hp and and full ability and you can stack many buff too if the hime has stacking frames. I must admit many veteran player doesn't really have many experience using different strategy to beat GO using weaker grid or glass canon build but its possible. Well even you can one shot getting one orihalcon is not bad too.