Don't use this:
Ludicolo @ Life Orb / Mystic Water
Ability: Swift Swim
EVs: 4 Def / 252 SpA / 4 Spe
Modest Nature
- Rain Dance / Focus Blast
- Giga Drain
- Hydro Pump / Surf
- Ice Beam
Why it's bad: This set is not bad and arguably still its best set. It's speed is very nice under rain and it has the ability to run a modest nature. But, personally, with the adding of Kabutops (who, run Adamant), it needs to be able to outspeed and OHKO it with Giga Drain. This has actually become REALLY important because Kabutops is a top-tier threat under rain.
Use this set when Kabutops isn't an issue. It also adds the ability to outspeed many common threats with a Neutral nature.
Use this instead:
Ludicolo @ Life Orb / Mystic Water
Ability: Swift Swim
EVs: 4 Def / 252 SpA / 4 Spe
TIMID Nature
- Rain Dance / Focus Blast
- Giga Drain
- Hydro Pump / Surf
- Ice Beam
Why it's better: Without Kabutops, this thing would NEVER run a Timid Nature. at +2, it can outspeed everything it needs to. But, it does not outspeed +2 >70 base speed or +1 base 100s and up. With a Timid nature and rain, it reaches 524 speed; which means that under rain, this thing can outspeed most common scarfers: including
Scyther, Pyroar, Raichu and Zebstrika. It can also outspeed +2 +spe Barbaracle (quite a niche) and other threats.
It also outspeeds Kabutops under rain; considering that every Kabutops runs Adamant. However, the power difference is noticeable. It depends on if you want to take out common threats easier or if you want more power. Also, the added speed helps it outspeed stuff under trick room (base 70 is ok)
Here are some calcs (done without SR for comparison of damage output):
252+ SpA Life Orb Ludicolo Hydro Pump vs. 252 HP / 252+ SpD Audino in Rain: 238-281 (58 - 68.5%) -- guaranteed 2HKO after Leftovers recovery
252 SpA Life Orb Ludicolo Hydro Pump vs. 252 HP / 252+ SpD Audino in Rain: 218-257 (53.1 - 62.6%) -- guaranteed 2HKO after Leftovers recovery
The power difference isn't the biggest change, but it can determine a 2HKO from an OHKO, a 3HKO from a 2HKO, etc.
252+ SpA Life Orb Ludicolo Hydro Pump vs. 248 HP / 100+ SpD Eviolite Togetic in Rain: 160-188 (51.1 - 60%) -- guaranteed 2HKO
252 SpA Life Orb Ludicolo Hydro Pump vs. 248 HP / 100+ SpD Eviolite Togetic in Rain: 144-172 (46 - 54.9%) -- 58.6% chance to 2HKO
The damage is rather noticeable, but it depends on how much you think the power difference matters.