Yes, by being in second place at the end of the standings, it proves that Lazio can compete with other big teams. And now their task is bigger because they have to play in the Champions League which has stronger competition than Serie A. You are right Lazio have to finalize their attack line, because no matter how strong their defense is if they can't score goals it won't make them able to win.
Lazio will be very doubtful in the Champions League, because there they will face other strong teams whose games are very different from the strong teams in Serie A, so it will be better for Lazio to focus more on competition in Serie A and other domestic competitions that the chance for them to win is bigger than in the champions league. It's a shame it's been a while since a team from Serie A hasn't done well in the Champions League this season, there's Inter but we still have to wait a few hours before seeing the results.
But I read the news that Lazio is quite active in the transfer market this time, their ambition is to be able to play well in international competitions and that makes sense because it would be a shame if one season's struggle was wasted because their squad was not sufficient enough to compete in the champions league