i have recently started playing around with endymion, it does feel like a complicated deck deck to all the text, but it is one where you can run a number of engines
in does work well with mythical beast, generating lots of spell counter to give me draw power, search power or to blow up the board.
start with your spell counter bank, this could be the field spell Magical Citadel of Endymion, Magister of Endymion, Servant of Endymion, Reflection of Endymion and Magical Abductor, or a mythical beast that you managed to special summon, using mythical beast jackal king. mythical institution, make a good middle ground.
your search are servant of endymion (special summons itself and a spell counter monster from deck), magical abductor (searches any spellcaster master from 3 spell counter), mythical institution (searches any spellcaster at the cost of spell counter equal to it level), mythical beast master cereberus (search any mythical beast monster).
draw engine is spellbook with blue boy, and spellbook of knowledge, from later game drawing the field card is added but is a flex card. another draw engine is sky striker, i original ran instead of red-eyes dark dragoon. yes red-eyes dragoon is not need to break board but i got fed up to losing to it. i did run accesscode talker, because i don't own the card but left it in the side deck option, the sky striker engine is a flex spot as it gives you access to powerful cards like widow anchor (to negate and steal monsters), afterburner (surgically breaker baord and backrow) and shark cannon (to disrupt graveyard plays), not to mention thin out the deck and give accesscode talker more material to work with.
red-eyes package because red-eyes dragoon is a thing and is to be feared or play around.
all the any card are a self explanatory, chronograph sorcerer, is a free monster on the field, grabbing timegazer from deck, special summoning itself and another speelcaster when i spellcaster is destroyed, great high scale at 8 and at level 6 easy to pendulum summon. it value increase once odd-eyes vortex is on the field as you can send timegazer back to the deck for an omni negate.
pendulum treasure is a foolish for pendulum, is you drop a mythical beast, you can use mythical beast bashilisk to put in back in deck and draw, jackal, is an low cost search for mythical instution, but is a plus in generating spell counter, garuda is backrow removal if needed, but can also bounce special monster back at the cost of 3 spellcounter, great for disruption, i wish i could run more, so any extra is a side deck option.
instant fusion, i have choose to flex with thousand eyes and millennium eye, millennium is a definite one of thousand not so much and would be my replacement for accesscode. i just like to fact i can steal an opponent monster and link it off.
in the extra deck as mention a lot depend on how you want to play and whether you have the cards or not, day-breaker is a flex spot and can be sub out for any link-2 as can abomination unchained, if accesscode is your thing then borrelsword, is also flex. i did sometime wish i had more then one selene but by then i have set a board and just want to extend more.
going first i usually end with apollousa with 3 to 4 negates with endymion the master himself, or 2 negate with endymion, and mythical beast king.
going second i have been a break board and playthrough about 2-3 disruptions, handtraps or otherwise.
i have not tried this build with handtrap, but i image you can run dimension shifter, as you don't need your graveyard, if you have your scale and field spell, selene would still get 3 spell counter and can summon from hand.
macro or dimersional prison, mystic mine and secret village can all fit in this deck as floodgate to establish the board or to stop opponent from playing.
hope you have fun.