First and foremost, this is a Ground type gym, which means Grass, Water, and Ice types sweep through this gym like a hot knife through butter. Dewott has the easiest time with this Gym, because you should have Water Pulse by this point of the game, both of which do heavy damage to these Pokémon, due to weakness and a lack of Sp. Defense. Servine should have Leaf Tornado to do the same, but be careful with accuracy. Be sure to pick up the Fresh Water from Clyde before advancing.
Onto the gym itself. Clay decided against his elevator puzzle in Black and White and made a conveyor belt puzzle instead. Every time you move to a platform, the lights turn on and you can now see the platform permanently. This puzzle is overly tedious because the camera is a tad too close. Luckily, there are some elevators that can show you all of the platforms you have lit up, so you have a good idea of where to go next.
Despite all of that, it does not prevent the gym itself from being gargantuan. There are 36 total platforms, and the platforms don't always link to each other easily. The pink conveyor belts go under a platform, so that also messes with progression. The fastest way to get through here is to take this path of platforms from the beginning: Up, right, down, right, up, up, up, right, down, right, up. This will lead you straight to Clay without any of the hassle.
|2||Tavarius||Drilbur 29||Baltoy 29||Sandile 29||1160|
|3||Niel||Baltoy 30||Sandile 30||1200|
|5||Pasqual||Drilbur 30||Baltoy 30||1200|
|6||Tibor||Sandile 30||Drilbur 30||1200|
|8||Clay||Krokorok 31||Sandslash 31||Excadrill 33||3960|
If you decided to defeat all of the trainers, then you are not going to feel like going through all of that conveyor belt madness again. If you just used the previous path to go right to Clay, then the trip is not as overbearing. Clay himself is in the limbo between easy and difficult, although that really depends on the placement of the team. If you challenged Clay immediately after Elesa with next to no training whatsoever, then you may run into some issues.
Clay carries Pokémon with higher Attack stats, and, on top of that, high defense (except Excadrill, who has the lowest Defense of all fully evolved Steel types.) Krokorok even has Intimidate to drive this point home. Servine and Dewott do marvels to Clay, especially the latter, as Steel does not resist Water, so Excadrill is still weak to Water. Pignite has some problems with Clay, with its Fighting moves at this point being all Physical, and leaving it in against Ground types with high defense does not bode well. Luckily, Arm Thrust hits multiple times, so the output is still pretty high. He should not be too much of a problem, but do not underestimate him. Clay's Excadrill also has a Sitrus Berry, just like the previous Gym Leaders.
After defeating him, Clay gives you TM 78: Bulldoze. Bulldoze is a 60 Power, 100% accurate Ground type physical move that lowers speed when it hits. Think of it as a cross between Icy Wind and Earthquake. Clay escorts you out of his gym, and you are free to leave and explore Route 6 and beyond.