Maeda Keiji:  Have a saved game file from Samurai Warriors.

Inahime:  Have a saved game file from Samurai Warriors: Xtreme Legends.

Hard difficulty:  Successfully complete the game under the normal difficulty setting.

Expert difficulty:  Successfully complete the game under the hard difficulty setting.

Continue game:  After successfully completing the game, you can start a second game with all magic skills, troop levels, and the weapons, helmets, and armor that were equipped at the end of the last chapter.

Unlock all officer biographies to start a new game with a troop level and experience bonus.
Unlock all music to start a new game with a gold bonus.
Unlock all battles to start a new game with weapon and armor bonuses.
Unlock all moves to start a new game with extra troop types.

Level up skills faster  -  This may sound simple but you would be surprised how many people miss an opportunity to do this.

First off in battles before you finish off the commander of the enemy forces or meet your objective you can use any skill slots you still have remaining. If your unit cannot use them on an enemy then just use a support skill on yourself or an ally. If an enemy is in range use whatever you want on the enemy. If they are low on hit points you could just use skills that diminish stats like defense or offense until you run out of skill slots. It's also best to take advantage of using your skills you are just learning, it will say the level of your skill when deciding which to use, so if you see LV 0 on any be sure to use them as much as possible.

Doing this will give you a chance to learn a lot of skills early in the game, giving you a chance to learn more advanced skills earlier in the game giving you a huge advantage.