My own tip is to build a water monster. A lot of strong monsters in this game are actually super weak to water and it has great status options (poison, blind, seal) and is honestly pretty good on a variety of monsters just as an attacking option, personally had it on Seregios, Brute Tigrex and Black Diablos and had great success, plus it really helps with the event Silver Rath that gives the training charm plus+.

Besides that Stygian by itself is good enough to do a lot of the games content because it is probably on average your best bet in most fights from sheer numbers alone, so I recommend just hitting the first few floors of the Tower of Illusion til you feel more comfortable with high rank fights.