Hello !
Firstly i've got to say three words : Lafate is God !
From my own point of view, and i'm quite new to this game aswell i started playing one month ago, you shouldnt focus too hard on this event since beatrice seems to be quite a nieche unit but the sailor can be a decent assist unit.
So if i was you i wouldn't try to clear as much map as possible before the end of the event like next week end to aim for 15-16 stars and for now focus on grinding and leveling your team in story mode.
You should follow this guide : http://harem-battle.club/millennium-...-1-2015-a.html
It helped me a lot and with a basic team of 50cc40 like i am right now i can almost 3 stars all the event.
You should aswell read the faq (if not done) : http://harem-battle.club/millennium-...questions.html
From the event point i guess you can get the 4 first map to 3 stars (12 stars) then you would need 3-4 stars more to be successful. The map 5 is quite harder then previous ones since sea monsters start to appear but you still got more then a week so you may be able to make it in time.
To make it in time i think you should try to get through castle retake (1 star smth) and grind on dragon hunting/remnant vanquish43 asap since it will provide you with decent units and a bunch of exp.
To 3 star the 5th map it seems according to many guides that you will need 2 CC healers and 2 CC archers. You might find a guide or a strategy cheaper to do so, give it a try on da web.
Appart from the event, i agree with the previous post of my comrade and i don't think iron units are any useful. But bronze units may be, especialy soldiers, archers, mages.
Good luck dude ! and gg to 3 stars the first three maps !
Edit : oh and i forgot but you may read this http://millenniumwaraigis.wikia.com/wiki/Guides
if i did myself i wouldn't have to ask Godfate a lame question !
Edit 2 : i think it could be a good idea since you can three stars some maps, to try a few runs to get an Alissa, if my memory is correct you don't have enough silver healers in standard missions to CC 3 healers, but from my own experience it seems that 3 healers (even if some are silvers) can be of a great help ! I wouldnt be able to 3 stars the machine gun sea monsters maps without'em !