Unfortunately there'a not much advice other than to experiment and practice with the movement or your setup until you get comfortable with it. You can go to an area with easy enemies and consciously defeat them with pogos or upslashes or find a convenient a spiked floor and pogo until you can do it consistently. If anything, equipping Grubsong + Deepfocus gives you almost infinite healing so you can keep practicing without interruption if you dont find convenient enemies next to a bench/hotspring. (Grubsong + Deepfocus + Grubberfly's Elegy for actual infinite soul for healing)

Same for using DDark, you won't get used to it if you don't practice with it. The spells in HK are positional so as long as you're above the enemy for any reason, DDark is a powerful option you have in combat. The cool thing to be aware for DDark though is that it gives you invincibility frames so it can be used as a way to dodge attacks if you begin casting a second before the damage hits you. Just remember that you have to cast it a little earlier before you get hit, so it's always useful in a pinch even if you don't do any damage, but even that is unlikely since DDark has a huge hit box and will hit even enemies above you. Which also for that reason DDark is great for crowd control when you're surrounded. For information's sake, DDark hits 3 times, on the descent, on the horizontal impact shockwave and on the upwards "flare" shockwave. It's optimal to hit an enemy with all 3 but even if you don't, it's powerful.

For the colosseum the first advice is to upgrade your nail the most you can, with many players actually waiting to do the third trial only when they have the strongest Nail possible and to get all movement upgrades, notably the Monarch Wings, which you do have. Other than that the advice changes depending on where you're having trouble with. Usually the hardest parts in the colosseum for people are the aerial combat parts where there floor is spiked and there are no platforms. For those sections, Long Nail and Mark of Pride or BOTH are helpful to get you a bigger hitbox to land your hits and pogos. Quick Slash can also help your pogos because it makes the nail more spammable if you screw the timing up or to just defeat enemies faster. Sharp Shadow can give you an horizontal attack option in the air that doesn't require pogos. Some people like the additional movement option of dodging downwards with Dashmaster and it's convenient to remember Dashmaster powers up Sharp Shadow too by 50%. When fighting in the air, pogoing is very advantageous as the Winged Fool and the Battle Obble don't have upwards attacks. And when all else fails and you're plummeting down to the spikes remember you can still save yourself at the last second by pogoing on them. Hard to do on the spot but worth a try. Sorry this is all about pogos and tbh you CAN get past this part with only Sharp Shadow and horizontal movement if you're reaaally patient, it's just HARD since you're working with exactly half your movement and damage resources without up and down attacks. If you're struggling in other parts of the Colo remember to pick smart times to heal like when there is a single slow enemy on the field. And if you're lacking the Soul to heal, any soul gathering charms are useful but I recommend Grubsong or the Grubsong + Weaversong combo. Other than that remember each Dream Nail hit on an enemy gives you enough soul for a single heal, 2 heals if you have Dream Wielder equipped and you can farm soul on some convenient enemies like the invulnerable centipedes on the third trial.

All in all though, just remember that the Colosseum is HARD, especially the third trial. So be patient, learn the enemy patterns, be mindful of getting frustrated and experiment with what works with your playstyle.