Slogra and Gaibon are two bosses you'll encounter early on in Vampire Survivors: Ode to Castlevania. Finding and beating these monsters inside Dracula's castle isn't that hard, and doing so with Grant Danasty unlocks the Sypha Belnades character for you.
If you're just starting with Ode to Castlevania, I will recommend you acquire the map from the east section of the map. It can be found inside the forest marked with the green arrow on your screen. This will allow you to get an overview of the stages, and more importantly, the location of the bosses. I've marked the location of Slogra and Gaibon on the map above along with the path to reach them (see yellow lines).
After you cross the bridge and enter Castlevania, keep walking west until you reach the first boss: Giant Bat. Bosses are marked with the skull icon on the map and you need to stand on the red sigil to summon them. Defeat Giant Bat to get the Stallion Gate relic and access to the next area. Follow the indicated route and you'll then reach Giant Medusa Head boss sigil. You can skip this boss fight if you want. Move further east and you'll come upon the sigil that summons Slogra and Gaibon.
Walk over it once to summon Gaibon and again to call in Slogra. To make the fight easier, I recommend you fight and defeat one boss and only then summon the next enemy instead of taking on both at once. You can collect power-ups and your preferred weapons before beginning the fight. Also, make sure to level up your arsenal to defeat Slogra, Gaibon, and the other enemies effortlessly. Keep an eye out on the light sources (chandeliers) as these drop useful upgrades. For instance, I managed to take out Slogra in a few seconds with the Nduja Fritta pickup, but you have to be very lucky to get one.
You can skip the first two steps and simply evolve the Dragon Water Whip if you find this weapon while playing Ode to Castlevania. However, I've noticed that this weapon is a rare find. Therefore, it's ideal to first unlock Trevor as you get the Dragon Water Whip by default while using that character.
First things first, go west towards the entrance of Dracula's Castle at the location marked on the map above (you can use any character here). There you'll find a coffin surrounded by bats that you can't reach easily (second image above), but your whip can. Take out all the guard bats and approach the coffin to release Leon Belmont and add him to your characters list.
Select Leon Belmont as your character for the next round in the Ode to Castlevania DLC. Level up to acquire some good weapons and upgrades and go to the Giant Medusa Head boss (see map above for location). Stand on the red sigil to summon the monster after which you can take it out. Because you have Leon equipped, defeating Giant Medusa Head will unlock Trevor Belmont for you.
In the next match, select Trevor Belmont as he comes equipped with the Dragon Water Whip as his base weapon. Now you don't have to rely on your luck trying to find this as a drop. The next step is to evolve the Dragon Water Whip. For this, you need to max level up the weapon and acquire the Attractorb magnet.
Once these two tasks of max leveling up the Dragon Water Whip and acquiring an Attractorb are done, you need to defeat any boss to get a Treasure Chest. Opening up the chest after fulfilling this requirement automatically spawns the Hydrostormer Tip. After you get this item, you'll have evolved the Dragon Water Whip granting you the Grant Danasty character.
In the next round, unlock Grant Danasty for 241 coins and select it as your character for Ode to Castlevania. His main weapon is the Curved Knife, which I don't like much, hence, I level up the character and acquire some other good weapons and upgrades before entering Dracula's Castle. As mentioned previously, all you need to do now is defeat Slogra and Gaibon to get access to Sypha Belnades. You can then unlock this character permanently for 2,165 coins.
For more on Vampire Survivors, check out How to unlock Christopher Belmont on Pro Game Guides.
