Upgrade each of a character's attributes evenly along the whole upgrade chain.
You won't be able to access many of the upgrades until you've made progress through the single-player campaign.
Spend your Soul currency whenever the jar's full, otherwise you won't be able to earn any more.
Souls allow you to take part in multiplayer battling, so make sure you're spending these on building your artifact collections.
You should keep your allies at the same level as your samurai, to ensure they're fit for battle as the challenge of the game increases.