Basically. Whenever i fight enemies my strategy is:
Build an ex combo for Sagi while randomly use healing magnus which shows up while i discard cards.
My deck has 45 cards in it :
1 weapon for each character 2 specials for Sagi and Milly 3 Specials for Guillo 2 mid combo cards for Milly 3 healing items for 1 target 3 healing items for party 1 Idol for ressurection 1 Force everything else are Attack and B Attack cards
I write this since i stumbled upon a problematic Colloseum battle where i have to kill 5 enemies who are fast as f**k and they just won't let me build any kind of combo...
General rule of thumb: whatever you're constantly having to discard because you run into it more than you need it, you have too much of that in your deck. Whatever you're constantly trying to get by discarding, because it doesn't come up as much as you want, you have too little of that in your deck.
My $0.02 is that you probably have more regular attack cards than you need. If I did my math correctly, your 45 cards consist of 18 that you listed and 27 regular attack cards, which would be 9 each of weak, medium, and strong. Even after accounting for regular vs. B varieties, that's probably a few too many. (Especially since you only need B for specific EX combos, so you don't need any more copies of those than you do of the corresponding specials.)
EDIT: Another $0.02: try to keep your total deck size to be close to the number you need to get the max amount of cards in your hand (which I think is 30? maybe 40? been a while). Before that, there's a definite benefit to bigger deck - namely, bigger hand too - but after that, the only reason to have more cards in your deck is if you actually want each and every one of those cards. The smaller your deck, the faster you cycle through it and the sooner any given specific thing will show up again.
- Vilurum Xaren, leader of the happy turtles, and knowitall. ... And if you can't spell "Vilurum", call me "Vil". :)
Just stack your deck with Sagi's apotheosis, Guillo's frigid queen's festival, and Milly's empyreal thunder with their godly equipment (vajra, deluge, and laevateinn). Put one or two Book of Manas and one force assuming you are not deck class of 30. Add 2 weak, medium, and strong attack B's. Add 3 weak, medium, and strong attack B's. Also, have milly's mirage turn and canyon wind. When the battle starts, start discarding for Guillo's combo while building mp with milly and force. Then use Guillo's combo and wipe the enemies off the face of the earth. It's that simple if you have gotten far into the game. If not. Guillo's ice queen, sigil cry, and zeniver;s cascade should be sufficient.