Any card that is from an event or rankings isn't obtainable anymore. The event gacha cards are still available (assuming not removed this next event gacha) however, you would have to wait for the event to start before being able to try to roll for and then would have to use money/your free e-tickets if you had any. Any card that says normal gacha or a stage #/ghq are still obtainable.
Depending on your current cards/sed you might want to try to go a more economy type of style until you can get what super rares you do have up to snuff as it were. In Coliseum your cards get a damage bonus against a certain type they are strong against. This is done by using Moe against Sexy who goes against Babe who completes the circle against Moe. A card gets a x3 bonus to their attack against cards they are strong against.
Using that method a babe card only needs to be at 3706+ to beat ANY current moe card including up to 5 ex ranks on Kanae.
A sexy card with at least 4012 attack will beat any current babe with up to 5 ex.
A moe card with at least 4379 attack will beat any current sexy card with up to 5 ex on it.
For just starting out, unless you do want to use a different type, moe is the fastest to level so can reach the needed attack the quickest the first 50 levels than slows down after. Babes level slower but evenly throughout all 100 levels. Sexy level slowest the first half but then quickly after level 50.
Using a more economy method you can even use rare cards if needed to try to complete a deck to go against a specific type of cards. It will be harder against people with trap decks and same type decks, however, as you get more cards and levels that can be dealt with in time.