If you feel like gambling, most Swamps only put in 140 EV's into speed (to Outspeed Scarf-Lando-T in Rain). So if you instead want to got Modest for more power in other situations, only put 148 into speed, and play for bulk with the rest. This build Will OHKO what it is designed to beat, but you will easily drain off any damage done to you if if it is faster or too bulky, where Swamps ice punch does the most damage.
I simply put the leftover EVs in Special Defense after investing in speed and Special attack. This was a choice to get more worth out of the Assault Vest and greater effective returns from Giga Drain, but you can choose to put the 108 into HP instead, if you want physical bulk. Scald is pretty helpful for that also.
If you really want bulk, you can modify the set even further to bare minimum Special Attack investment for the important KOs. Did come calcs for the main threats that Ludicolo is for (based on what you said):
- 156 SpA Ludicolo Giga Drain vs. 116 HP / 0 SpD Mega Swampert: 192-228 (101 – 120%) — guaranteed OHKO
- 8 SpA Ludicolo Ice Beam vs. 4 HP / 0 SpD Landorus-T: 168-200 (101.8 – 121.2%) — guaranteed OHKO
- 220 SpA Ludicolo Giga Drain vs. 252 HP / 148 SpD Rotom-W: 84-98 (53.5 – 62.4%) — guaranteed 2HKO after Leftovers recovery (all sitrus berry users will be 3HKO, D:)
Mega Swampert in the Rain hits hard! It will likely heavily damage if not KO Ludi's partner if you can't bring it down.
One typically has to protect the previous turn with whatever partner to safely switch in Ludi beside it, So the smart player will go for the exposed partner. Oddly, switching swamp on sight of Ludicolo isn't common in my experience with the scenario, they don't want to waste limited rain time I assume.
So in theory, you could drop to only 220 EV's Special attack to allow for more defensive investment. This gives you 140 EVs to place defensively, Where I still recommend Special defense exclusively.
In the end though, Giga draining proper targets for more damage really helps your bulk as well.
- [0/0/0/252/84/172] Timid to outspeed any adamant M-Swamps — 252 SpA Ludicolo Giga Drain vs. 4 HP / 0 SpD Mega Swampert: 192-228 (109 – 129.5%) — guaranteed OHKO
- [0/0/0/252/108/148] Modest to outspeed common M-Swamps ONLY. — 252 SpA Ludicolo Giga Drain vs. 0 HP / 0 SpD Mega Swampert: 204-244 (116.5 – 139.4%) — guaranteed OHKO
Again, this is all for a Rain counter/Bulky Fake Out Ludicolo I built for a recent team. It also performs very well against the common bulky waters of the meta such as Milotic/Rotom-W/Jellicent/Suicune.
This leads to a final spread of [0/0/0/220/140/148 Modest].
Slightly more bulky than the other set, but deals enough damage when it counts. I still run the [0/0/0/252/108/148 Modest] On my team though.