Metal Gear Rising is meant for generally more casual-oriented play than scoring. Most of the time getting an S-rank involves just getting no damage and some zandatsus while completing encounters in a reasonable timeframe, the first being straightforward while zandatsus aren't terribly hard to pull off.
The ranking system doesn't make much sense either, it ranks you for your highest hit combo, but if you wanted to get the most hits all you'd need to use was the polearm. The BP requirement doesn't make much sense as it is an extension of other actions in the game such as simply killing and getting zandatsus, but the BP requirement for an S-rank on some bosses mandates you must pick up items you already have a full-stack of while slicing objects into as many as pieces as possible.
At that point the scoring no longer feels like it grades your play, but completing random tasks which have little to do with the gameplay itself. Meeting the combo hit requirement for an S-rank on some bosses is also rather ridiculous, as it usually involves going into Ripper Mode and slashing the boss as much as possible while in Blade Mode as well, because less energy is consumed in a simultaneous Ripper and Blade mode than just a normal Blade Mode. For the Sundowner fight you have to dodge around him all the time to keep up the combo instead of slashing off his shield, as most people would. In Bayonetta the scoring largely relied around not taking damage and getting your combo going, but maintaining a high combo in Bayo was more than just counting the total amount of hits.
The total kill requirement doesn't make a lot of particular sense either as you can't proceed without killing everything in the first place. But tricking enemies into killing eachother (which is even harder to pull off than with your moveset) does not count as a kill. Furthermore, you can't instakill B.D. Mistral on Revengeance difficulty by parry countering her ASAP and get to B.D. Monsoon faster with less fuss without losing your S-rank, as you're expected to kill some of those tripod fucks to inflate your killcount before you're able to get an S-rank.
There are no restrictions whatsoever on using subweapons, wigs, Ripper Mode, or even the Fox Blade to gain S-ranks, effectively making the challenge one of self-restriction which is stark to contrast to most Platinum games and their ranking systems. Some swords like the Armor Breaker and Stun Blade turn the game more into RNG where you keep spamming attack and move in for an easy kill when the sword's special effect manifests itself. An argument can be made that subweapons can be OP, as you can skip entire boss phases with the Pincer Blades, but the Pincer Blades do require considerable time to attack with.
You don't necessarily have to know your entire moveset to get good. All you need to know is to slide into grunts, slide into longsword guys and strike their sword while in Blade Mode for a free stun, being able to execute a Raptor with a charged sai and four light attacks, repeatedly stunning a GRAD with a charged sai, and baiting Gekkos into their bullrushing attack, though it's more like waiting until the AI actually decides to bullrush. The rest largely comes down to parry countering enemies on your first shot (on Revengeance difficulty) to avoid getting overwhelmed as a group of Mastiffs is beyond painful to handle if you aren't going full Ripper or using red phosphorous.
There's more fun to be had in MGR when you aren't chasing S's, because the slightest fucking bullet will ruin your no damage bonus and enemies like Gekko's, Sliders and Raptors love to fire rockets and a bunch of bullets at you which don't even deal serious damage, they're doing it solely to ruin your S-rank. Ripper Mode is painfully unbalanced as it's basically a 'fuck everything' button as it breaks armor like it's fucking nothing. The whole way zandatsuing is set-up is to keep you going despite the damage you take. If you want a more puritan challenge with less necessary self-restrictions, you should be playing as Jetstream Sam instead.