This will be a pretty normal, straightforward battle. As usual, concentrate on using your strongest Summons as the battle starts (especially Mars element ones). If you want, keep your Mars Djinn Set so that Jenna can heal with Aura spells. The Aqua Hydra is not likely to down any of your characters, so don't worry too much about healing. Just keep attacking the beast relentlessly and healing until it goes down.
The Serpent is your toughest Boss yet, but still not too hard. However, note that unless you have reflected all 4 beams of light on it, the Serpent CANNOT BE DEFEATED. Using his powers of darkness, the Serpent will fully heal itself every turn. Anyway, lead with Summons as usual, especially Jupiter ones. Also, note that the Serpent attacks twice every turn. Keep Jenna healing with Aura spells the whole battle, and make sure to have some Water of Life.
Come on, Camelot, you have to be able to do better than this! Your fight with the Avimander should be no longer or harder than a normal battle. As usual, have everyone lead with strong Summons. Have Piers Standby two extra Mercury Djinn and Jenna Standby two Mars Djinn so she can Summon Ulysses and still heal. The next turn, just attack the Avimander with your strongest Psynergy and he should go down.