Simple math: HP water + rain = 140 base power (assuming perfect IV which would not be the case outside of PO) * 2 = 280 base power against something SE on it coming from a pitiful base 60 special attack and mosly uninverted meaning it will be like 150 special attack at lvl 100...A few things could happen, though:
-Your Water Type is too weak to survive another hit (or worse, KO'd)
-You could be Burned by WoW or a damaging Fire-Type Move on a switch
-Pokemon with Intimidate could drop your Attack a few times and possibly carry Fire Type Moves (Arbok, Arcanine, Tauros, Mawile, Salamence, Staraptor*, Luxray, Stoutland)
-You face a high Defense-low Sp.Def Fire Type (Torkoal or Magcargo)
-You face a high Defense-low Sp.Def Pokemon that resists Bug Type Moves in-general (Aggron, Skarmory, Steelix, Golem, Gigalith)
-Cofagrigus enters hoping to screw you out of further Bug-Type Move boosts or Critical Hit blocking
-Counter and Metal Burst users could 1KO you from Megahorn use
-You could miss entirely (Megahorn's accuracy is 85% and you could be further hindered by Muddy Water, Mud Bomb, or a lucky enemy's Acupressure)
Protecting your Attack from hindrance is as important as KOing any Pokemon that can hinder it. Without your key offensive stat, you will have a ton of issues. Rain + HP (Water) is a safety net.
* Doesn't carry Fire Type Moves outside of a random Heat Wave, but it's very unlikely.
non effective STAB megahorn (which you can further boost with swords dance) 180/2= 90 base power from 135 base attack stat his attack stat is mostly inverted and have a chance to be boosted to 180 power again
85 accuracy is more than acceptable (8.5/10 chance to hit).
turn 1: you switch in excavalier in choiced latios with draco meteor (or what ever)
turn 2: they switch to volcarona then you swords dance
turn 3: they use quiver dance cause it sounds stupid that you will stay then you KO with megahorn.. (and if there is stealth rock better)
iin conclution special=useless