I’m not nearly as experienced as Adrianus, but I think either dicens or dicendo would be grammatically correct. They’d just convey slightly different shades of meaning. Disregarding for the moment the tense of the infinitive, what you wrote, using “dicens” would translate as:
Marcus, saying that he had been attacked by a bull, tries to excuse himself.
(dicens, present participle, as an adjective modifying Marcus)
What Adrianus was suggesting using “dicendo” would translate as:
Marcus tries to excuse himself by saying that he had been attacked by a bull.
(dicendo – ablative of means, using the gerund, a verbal noun formed from the neuter future passive participle of the verb.)
In other words, even though in English, we use “saying” in both sentences, in the first sentence it’s functioning as an adjective (modifying Marcus) and in the second as a noun (modifying nothing and in the ablative in the Latin sentence because it’s expressing the means by which he is trying to excuse himself.)
The infinitive, as Adrianus said and as you appear to have understood, should be “pulsatus esse”, the perfect passive infinitive.