In advents that's not the case, so if you can't farm ultimate consistently on your own, you might not be able to finish everything. On the other hand, you don't have to finish everything, since you can decide on what to get. Therefore it's important to set priorities, make sure you manage to max out whatever is most important for you instead of trying to get everything and ending up with everything half finished. If you can farm ultimate and got a decent enough pool of half elexirs, grabbing both the weapon as well as the eidolon shouldn't be too much of a problem though.