Random execution method

You will need two different methods of randomization to do this. The first uses percentages, and the second uses random integers.

Make a procedure with the global trigger 'before entity is hurt.' Then use an if bracket to check if the entity is your desired entity type. If it is, you're then going to need to use the 'Do with 70% chance' procedure template. (You can find procedure templates in the upper left-hand corner of the code builder.) The idea of this is that it generates a random decimal number between 0 and 1, and checks if the number is below a certain value, only running the code inside if it is. (a value of 0.7 means that whatever you put inside will run with a 70% chance. You can raise or lower this value as you require.) 

Then, you need to randomly select which potion effect you want to apply. Nesting a bunch of these percentage random chances can make it very difficult to determine the actual probabilities of things.

Instead, make a local number variable for your procedure, (I'll call it 'potion_type,') and use the 'random integer' function from the math tab to set this random variable to a number between one and the number of potion effects you want to be possible. If potion_type equals one, apply the first potion effect. Otherwise, if it equals two, apply the second potion effect... and so on and so forth. You could also use random integers to randomize the duration and level of these potion effects, if you wanted. 

Anyways, hope that helps. I can send a screenshot if needed.