within the limit of his/her knowledge.
Quran has not said that Only person with a particular position/degree will give Dawah. Alim/Non Alim all will give Dawah but according to his ability and knowledge. Even an Alim has a limitation if he will start giving verdict like a Mujtahid/Mufti/Imam he is crossing limit. Similarly if a normal person start giving Masael/Islamic Rulings/interfering with Ulema jurisdiction he is doing wrong
2.There is no limit of Knowledge.No one can say that this much time/this much syllabus is having complete knowledge of Islam.
Quran and Ahadith has not specified the limit of knowledge.