I assume by playing around Crimson Blader you know to drop your
earth/fire color first, or to have sacred sword/veiler ready to avoid
getting bladered. Be aware experienced players may wait until the second
summon or later depending on their perception on when you can play around
it or when you have to give them extra draws to not get locked.
As rulers have developed there are scenarios that have come up because of various techs players may or may not run, examples including
Tributing dracosack immediately to avoid veiler, leading many players to veiler on summon, which also happens to get around Enemy Controller being chained to veiler to both dodge it and steal a monster, (note: anyone playing around veiler by tributing immediately does not likely have the Enemy Controller atm, and seeing them change that play may indicate they have recently drawn it, assuming you've discounted other factors.)
Not dropping tokens at 2600 or less life, as gaia is game.
Teching LADD to steal the mirror from the unprepared, though this will Backfire hard if they are, as chain link 3 response to LADD will work, negation will lead to them outing, taking, or killing through it.
Summoning dracosack is defense when they have Maxx "C" in case the opponent has Forbidden Chalice and a way to make colossal, which is especially prevalent if LADD is a popular tech.
Summoning extradeck monsters that are not dragons/machines to play around Electric Virus, often mentioned when trying to keep vanity alive.
Keep in mind:
Tokens don't go to grave to trigger blader, blader doesn't work in a crash, and gaios with material stops level 5 and under from attacking, and doesn't just negate effects.
Armory Arm and sacred wyvern are common synchros, try to make sure your current field does not result in you dying to them.
Decklists run 20 rulers, 1 tuner, 6 hand traps, 3 sword, 2 sarc, and probably 3 super rejuv with staple tech like storm/BoM, 3-5 tech, not much higher. Expect Hoban's vanity's and Card Destruction but don't be surprised to see things like e-con, phoenix, psi blocker, Dark Hole, droll, scarecrow, Return, LADD, Obelisk, EEV main/side. There's all sorts of random tech that steal games from the unprepared, keep track of the tech so you know what to expect from this player/deck, this format loves to give games to the unexpected trick.
3 of each ruler, 2 of each baby, and each is hard OPT, keep track of who has been sued that turn and how much is still in deck while not forgetting sarc actually adds a card in 2 turns.
If you do not have veiler, any Big Eye with a material is basically a blank to a Big Eye, any 2800 beater is a blank to collossus, (note the number changes based on the warriors in grave, which are crimson and collossus,) don't forget this with deckout by maxx math, sometimes just going for game is the simple answer, especially if under maxx where you are outclassed the instant they get a turn.
Rulers are a deck of many hard OPTs, 3 veilers, and a couple tech, if their system is fully online and not short on stuff at all the number of cards they draw might be fluff, even worse if their babies lack rulers in deck to drop. (Do this when you know their techs/veilers have been drawn/used, this is basically a very lategame gambit.)
While Blaster will not fall to blader because of the crash rule, it being blanked by collossus and being a nice veiler-proof out to something like vanity means if you can hold a blaster shot in your hand, it is really nice to do so. Rulers have hand effects too.
Memorize Hoban's list, card for card, his build is the most likely ruler deck, reinforced when seeing his techs like vanity's, just don't forget the previous point if you see something else, people had their favorite techs too.
Time was a well-known fact of the format, make sure you don't lose on it if it is a factor in your metagame as well, thought ruler, gaia, and Armory Arm are famous abusers of this fact.