Well, Kusare Gedou to Chocolate is both very fast and, if sung in the original key, very high (depending on what vocal range you have). For the speed, just reading the lyrics and practicing should get you going in no time at all. If you’re listening to the song on YT, you can slow it down and keep increasing the speed until you’ve got it down. Pitch wise, if necessary, you can lower it to wherever it suits your range best. You can get more comfortable range, but you can’t natually change it (there are certain surgeries and pills, but I wouldn’t go there). Raspberry*Monster is also very dependent on a good range. Nailing some of the higher notes might be harder if you have a more prominent low range. If you’re going to try to go for the high ones, then make sure you’re using good breath support. Don’t try to tackle those with your chest voice (it could potentially cause some injuries). When it really comes down to it, most songs are hard because of either the range, rhythm/speed, or both. Of course, there can be more factors, but these are two big ones you’ll encounter, especially in VOCALOID songs. If you want me to elaborate/if you have any more questions, don’t be afraid to ask me!