Higher Star Tera Raids require set-up.
In general, it's hard to solo 5 star+ raids without training your Pokemon to handle Tera Raid battles. While there may be a few stray 5 star raids your playthrough team can handle, even at level 100, Tera Raids can be challenging if you don't take the time to EV train, IV train, and adjust your Pokemon's items, nature, moveset, and Tera Type accordingly. I won't make a guide on how to competitively train your Pokemon, but for starters, use EV reducing berries or Fresh-Start Mochi to get your Pokemon to 0 EVs for a clean slate, and using Bottle Caps, you can talk to a guy with an Abomasnow at Montenevera to undergo Hyper Training and max out a Pokemon's IVs. When you have 50 Tera Shards of a specific type, you can use those Tera Shards at the Treasure Eatery in Medali to change your Pokemon's Tera Type.
Despite this, a majority of 5 and 6 star raids can be solo'd with relative ease depending on the Pokemon you use. Belly Drum users like Azumarill and Iron Hands especially are great for maxing their attack, using moves like Drain Punch or the Shell Bell to recover HP, and doing massive amounts of damage quickly. Pokemon with set-up moves like Swords Dance and Nasty Plot are also easy to use, while defense lowering moves like Screech are useful whenever the raid boss's shield isn't up. Pokemon with moves with secondary effects like Acid Spray can affect the raid boss even with the shield up, Abilities can boost your damage outputs with strategies like Facade Guts or Contrary Leaf Storm/Superpower/Tera Blast, and moves like Rage Fist can rack up damage very quickly. Items are important too, with Shell Bell providing healing when your Pokemon lacks a recovery move, Safety Goggles providing safety from Spore, and Covert Cloak blocking additional effects of moves, just to name a few. Depending on your Tera Type, Terastalizing can give you the power boost needed to deal extra damage and muscle past the boss's raid Shield, and using Cheers can be crucial when your team needs support or healing.
This doesn't mean a single Pokemon can solo every raid, as different raids require different strategies, so having a variety of Pokemon you can use in raids is useful, depending on how desperate you are to beat them. I'm won't sit here and list every single raid strategy, so I heavily recommend looking up guides online, YouTube videos/channels that feature Tera Raid builds, and even posts from sites on Reddit to get inspiration for different builds you can use. I don't main Tera Raids, but I find this website in particular to be very useful whenever I'm building a set for a Tera Raid.
7 Star Raids are event raids that are far more challenging than 5 and 6 star raids, featuring level 100 Pokemon with perfect IVs as well as specific movesets and raid scripts that heighten the raid's difficulty. These raids are harder to solo and require specific strategies, so you can't just go into these with your playthrough team or Iron Hands and expect to win. Since these are event raids that show up occasionally, featuring different Pokemon each time, there's no one Pokemon or strategy that is guaranteed to beat them all. While it isn't impossible to solo these raids with the right strategy and move order depending on the boss's raid script, these raids are often easier to beat with a coordinated team as you have a wider array of strategies and play styles to use. This holds true especially for the harder raids that rely on you rolling specific NPCs if you're flying solo. Each raid has its own strategies used to beat it, so whenever a 7 Star Raid rolls around, my best advice is to look online on YouTube, sites with 7 Star Raid strategies, and in larger Pokemon Communities for ideas of what Pokemon to use or for teams to help you beat the raid. It's much easier coordinating with people you can chat with than with random idiots online who don't know what they're doing.
I wish I had better advice to give than telling you to look up a strategy online, especially for these 7 Star Raids, but since they vary in difficulty and strategy, it's hard to give more specific advice. For 5 and 6 star raids, if you're just trying to get Herba or something, I personally recommend building a Pokemon like Iron Hands that can solo a bunch of raids and only focus on the raids you can beat. Once you have the resources you need to build better Pokemon (Money, Bottle Caps, EXP Candies, EV training items, Vitamins), that's when you focus on getting a variety of raid battlers and specific Pokemon for the harder raids.
Hope something here helped!
