If you get first turn, start by placing a pumprincess. This is so that you don't get attacked right after it gets destroyed. Otherwise, use mask on the current face up monster card your opponent controls and put down either a cobra jar or cocoon. Next, you can either summon Mefist by flip summoning the cobra jar and use the monster card (instead of the summoned token) to use as tribute fodder, or put another cocoon to be safe. Careful when using Cocoon since it is weak to either Enemy Controller or Windstorm of Etaqua.
Using the Jade Insect Whistle
Make sure to check how many Parasite Paracide are in your opponent's deck when the duel starts, so that you know how many whistles you can use. Use it when the current top card in your opponent's deck is not the parasite. Some people might put other insect monsters to counter this card, but it's still recommended to use this early.
When going against Bandit Keith with "Switcharoo" skill, they have the option to use the skill to shuffle their deck. They can put a 4-Starred Ladybug of Doom in the deck to counter the whistle. Once they draw it, they will often use Switcharoo to put it back into his deck. It's always better to use this after they used up both Switcharoos. Bandit Keith players also have a chance to draw the parasite by using their skill, this will make the parasite card's effect not go off and stay in their hand instead.
Aside from the parasites, this deck also has mask and burning land. You will need to choose carefully which one to go for since you only have 3 spell/trap card slots. One of them is also occupied by Pumprincess the Princess of Ghosts. Use these cards slowly to avoid losing everything to tribute summon or Straight Flush.
This card can be useful when you want to destroy your opponent's field spell like "Umi" or Harpies' Hunting Ground. These field spells need to be taken care of pretty quickly, while also saving your twisters for other cards,. It's important to know that this can also burn you as well, if you take more than 500 damage more than your opponent, then you will burn out faster.
Mask of the Accursed
When your opponent summons monsters that you can't handle, use this card to protect yourself. It's weakness is when your opponent start using tribute summons or cards like Order to Charge or Enemy Controller. Pay attention to your opponent's play style and see if they are going to ever tribute summon or not. If they use Thunder Dragon or have cards in their extra deck, it could mean they have high level monsters. Wait until they do, and use it on them.
Coming from a guy who ranked 760 in KC and just got plat 3(didn't try KoG yet) only playing parasite, I wouldn't use 3 whistles nor 2 twisters. Throw in another kuriboh due to the synergy with the panda.
Adding another very important point to my comment above, dark possessed soul is perfect against Relinquished decks.
Also, Inaba Rabbit as someone mentioned below is pretty good if used with burning land and cocoon.
I ran a similar deck, but i used inaba white rabbit instead of Mefist. Witnessed so many rage quits :D If you play it smart and save your masks for effect monsters or heavy hitters you'll love this deck. Also rabbit had an extra trolling effect.