For 5th seal, just get a PR/VR Wukong as ally, the rest is simple grinding.
If it's just to pass 5th Seal, Persephone + the other 5* Light cards + Wukong should be more than enough.
Your card pool, however, is old and very limited in terms of passing new stages - there's not much that's worth training given the current state of the game. There's a limited time seal in the Shop now and a new limited seal coming up next week that you might want to spend some diamonds on for more up-to-date cards.
Of the cards you do have, the ones I'd suggest training are
- Princess of Troy (Cassandra): Lasting effects are very powerful, and after PR she can be deactivated at will. Used in a lot of endgame setups.
- Gretchen and Thomas: Both decent universal boosters and team leaders, given you work within team restrictions.
- Sean: Always useful to have PR Starters.
- Mufasa: Frequently used burst card. Fits in most Earth and Beast teams, and a core member of the endgame Barbara-Nidhogg combo.
- Verdandi: After PR she's a core member in many Wukong-based grind teams.
- Dagon: Easy heart converter and adds HP to a lot of Water teams. After PR you can use it as leader, which also enables you to use PR Nidhogg as your ally without too much danger and opens up more powerful Dragon setups (Nidhogg's HP increase lets you grind through a lot of stages).
- Dark Dragon Spiritor (Novalis): Core to any endgame Dragon setup except Barbara-Nidhogg, but requires the Water Spiritor Ursula to reach full potential. You can often ally her until you draw your own so that's less of an issue.
- Karasutengu (Ootengu): After PR, a great defensive leader and a nice Earth member.
- Yuki-Onna: A stronger heart converter than Dagon, but more difficult to max out. Becomes a great defensive leader like Ootengu after PR.
- Wawel: A good member for Dark teams after PR; its HP and active are quite useful. Decent filler for Dragon teams too.
- Tefnut and Osiris: Useful when PR and amel IV as boosters and board cleaners (removing negative effects like weathered runes).
For now you can probably try some Intermediate and Advanced stages in the Lost Relic with a Dragon setup, like Dagon | Phantomasaur | Flamisaur | Novalis | Wawel + Dragon (Nidhogg is common but the HP drain can be difficult to manage).
Thomas | Urd | Verdandi | Valkyrie | Sean + generic God/Human leader (Li Hsiao Yao is good) can be a reasonably powerful team too.