As you are likely to know, every time you level someone up using Bonus Experience, three of their stats will increase. However, it is better to save your Bonus Exp for a character's later levels, when they have maxed out some of their stats. This way, when you use it on them, stats that they normally wouldn't gain much increases in will suddenly start skyrocketing.
For example, Micaiah is almost certainly going to max out her stats in Magic, Resistance and Luck (if Light Sage) very quickly, but lacks in some of the other stats, most significantly Speed, as she's usually too slow to double attack most enemies. However, when her Magic, Resistance and Luck are maxed out, start using Bonus Experience and watch her Speed rise.
When you use stat increasing items (Talismans, Secret Books etc), be sure to use them on a stat that a unit is lacking in. For example, using a Talisman on Ike will increase his Resistance, which is one stat he's lacking in. However, increasing his Strength would be a waste of time, as he's likely to max out his Strength stat by naturally levelling up.
When a unit is at level 20, you can use a Master Seal (if they are going to level up to second class) or Master Crown (if they are going to level up to third class) to have them change classes instead of using Experience. Since they are at level 20, you won't be missing out on any stat increases anyway.
There is a skill just like Nihil, except people fight without bonuses from supports, weapon triangle, terrain, and skills. Good for skills, because it can nullify the annoying Aurora skill from the Auroras. With this skill, more people will have a chance to help out kill Ashera faster.
Remember the holes in Chapter 3-11 that was such a pain to go over? You can easily go over them, requiring Airborne Units. Put the Airborne Units over where the hole was suppose to be if the trap isn't activated yet, and then let ground Units walk past them, unharmed. Keep doing this until you want the people to cross to cross.
This hint will explore the bonuses you will receive by transferring data from Fire Emblem: Path of Radiance to Fire Emblem: Radiant Dawn.
Note #1 - To do this, you must enter a Gamecube memory card with a Fire Emblem: Path of Radiance Epilogue save present. When starting a new game (also includes starting a new game on secondary playthrough saves) you will be given the option to transfer Fire Emblem: Path of Radiance data into your new game from the Epilogue save.
Note #2 - The North American regional version of Radiant Dawn apparently suffers from a glitch that when you choose to transfer data if you have an Easy difficulty save present (beaten or not) on Path of Radiance, your game will crash and freeze. To avoid this, either delete all Easy difficulty saves from Path of Radiance, or send off your copy of Radiant Dawn to get fixed (or don't load the Path of Radiance data. This glitch is not present in any other regional versions of the game, as far as I know.
Data transfer bonuses:
Expect most returning characters to have +5 added to their HP and +2 added to all other stats except Luck. Their weapons levels will also have improved.
You can view any Support conversations from the Path of Radiance Epilogue file that you have viewed.
Characters will have more Bond Supports, typically with characters they shared an A rank Support with in Path of Radiance.