I've been using this set for a while an it works pretty well in OU Assault vest roserade nature modest 20hp 192Sdef 252special attack and 40speed it's so specially bulky moves gigadrain sludgebomb hidden power fire dazzling gleam
This is a decent set, but the way you've written it out makes it fairly difficult to read... There's also a few small problems such as four missing EVs and no specified ability. That's okay, you're new, but there's a couple of ways that people here normally write sets. The most common one is the Pokémon Showdown format, which is recognised by the simulator and can be copy-pasted directly into the teambuilder:
Roserade (M) @ Assault Vest
Ability: Natural Cure
EVs: 252 SAtk / 192 SDef / 40 Spd / 20 HP
Modest Nature
IVs: 0 Atk
- Giga Drain
- Sludge Bomb
- Hidden Power [Fire]
- Dazzling Gleam
However, there are plenty of other formats that other people (such as myself) use, either out of habit or because they find them easier to write/read... Here's my method:
Roserade @ Assault Vest
Modest
Natural Cure
252 Sp. Atk/192 Sp. Def/40 Spd/20 HP (0 Atk IVs)
-Giga Drain
-Sludge Bomb
-Hidden Power [Fire]
-Dazzling Gleam
There's only a few slight changes in order and wording, as well as removal of spaces, but feel free to find whatever method suits you best, as long as it's easy to read and understand. The other perk of this layout is that it doesn't have to be concrete, due to it's incompatibility with Showdown; you can slash abilities and moves much more easily as it's used more for preliminary sets (like this one). Anyway, the set: As a specially defensive tank, Roserade faces a lot of competition from Mega Venusaur, who has far better physical bulk and can use status moves. They share counters as well; Talonflame, Mega Pinsir and both Charizard Megas can easily force Roserade out, buying them free turns to set up. Moreover, many of the Pokémon that Roserade has an edge over are physically based (such as Azumarill and Hippowdon), and can hurt Roserade a lot on it's minimally invested (and therefore frail) physical defense. For this reason, I'd suggest changing the EV spread to 252 Sp. Atk/216 HP/40 Def. This gives you better overall bulk and bolster your physical defense a bit more. You don't really need any investment in Sp. Def and Roserade's natural bulk and the Assault Vest are more than adequate for taking neutral and resisted special hits. The physical bulk, on the other hand, make you much better at taking hits like this:
252+ Atk Huge Power Azumarill Play Rough vs. 20 HP / 0 Def Roserade: 127-150 (47.7 - 56.3%) -- 85.9% chance to 2HKO
252+ Atk Huge Power Azumarill Play Rough vs. 216 HP / 40 Def Roserade: 120-141 (38 - 44.7%) -- guaranteed 3HKO
252+ SpA Roserade Giga Drain vs. 252 HP / 4 SpD Azumarill: 314-372 (77.7 - 92%) -- guaranteed 2HKO
(Giga Drain is usually better than Sludge Bomb in this instance because of the recovery that Roserade badly needs)
252 Atk Aerilate Mega Pinsir Quick Attack vs. 20 HP / 0 Def Roserade: 276-326 (103.7 - 122.5%) -- guaranteed OHKO
252 Atk Aerilate Mega Pinsir Quick Attack vs. 216 HP / 40 Def Roserade: 260-308 (82.5 - 97.7%) -- guaranteed 2HKO
252+ SpA Roserade Sludge Bomb vs. 4 HP / 0 SpD Mega Pinsir: 172-204 (63.2 - 75%) -- guaranteed 2HKO
(There's no way you're surviving a Return, but this is just for the sake of argument)
In all honesty, though, I think this set would work a lot better in UU, where Fairy coverage goes a lot further and you don't need to worry about Talonflame.