So far the only thing different about Easy and Normal Mode is the little fountain that fully restores Servant's HP/MP and the Master's MP making it much easier to grind.
Having it in either mode doesn't affect the Storyline at all, just Easy Mode gives you a fountain to heal you to make life easier. I recommend to use Easy Mode with Caster because of her low Physical strength and you need to grind as much as possible.
Plus have Caster at least two more levels about the Target level before the end of the week.
Multiple playthrough and/or fighting the same enemy time saves their Attack Pattern so it becomes easier to fight them to the point that you don't have to worry about healing your Servant anymore.
Hope that helps. --- "Invoke... VIRUS SOUL!!!" ~ From my VirusChris manga project PSN: VirusChris / JP PSN: UirusuKurisu / XboxLIVE: UtlimVirusChris
You can only change the Game's Difficulty level by either at the start of a New Game or beat the game and start a New Game+.
If you want to have Caster as a Servant, but you started on Normal Mode you can just go back to the beginning, on a New Game, and re-select your Difficulty level or you could plow on through with Caster if you want.
It's better if you know the enemies pattern by remember their patterns, writing them down, or view a guide. It helps when Casters kill her enemies without getting hit.
Use the Fate/Extra wikia here for some help (Please Note that it's in Japanese, but if you have Google Chrome you can have it translate the text to English, though not accurate, but it helps greatly for first time players):
http://www35.atwiki.jp/fate_extra/ --- "Invoke... VIRUS SOUL!!!" ~ From my VirusChris manga project PSN: VirusChris / JP PSN: UirusuKurisu / XboxLIVE: UtlimVirusChris