I did the research of what monsters are good for each role but what I wanna know if what role should I be focusing on? Is it worth having a sentinel or synergist monster? What role is best to focus on?
"Best to be single and live all alone Than to be married, and nagged by a crone"
The best com monsters are much stronger coms than serah / noel can become so definitely get one (or both) of cactuar / chichu / your choice. Serah/noel also can't learn offensive buffs, so synergist monsters are also something very helpful to have (can also use monster feral links for this though). Monster sens are helpful for normal playthrough and DLC but not required for main game with reasonably developed serah / noel. Goblin chieftain is hard to beat for economy of construction. Monster meds / sabs are of situational use only. Monster ravs are ok (esp. cloudburst) -- up to you how much effort you want to put in to make a good one.
definitely get a strong com monster definitely get a good syn monster sen monsters can be helpful at certain pts of game -- goblin chieftain highly suggested skip med / sab unless you want one rav monster is nice to have - up to you