I teach the subject of al-Aqeedah (creed) in Holland to new people. But there is another man who teaches them Sufism and al-Kalam (theology) and speaks ill of Ibn Taymiyah . What should I do?

Praise be to Allah, the Lord of the Worlds; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.

No doubt that learning/teaching 'Ilm al-Kalam (scholastic theology) constitutes a form of misguidance and evil. al-Salaf (predecessors) and Imams of religion dispraised and spoke evil of this knowledge and prevented learning/teaching it.
Abu Yusuf (May Allah's Mercy be upon him) said: 'Whoever searches for 'Ilm al-Kalam becomes irreligious' . al-Shafi'e (May Allah's Mercy be upon him) said: 'I believe that the best judgement against the people of 'Ilm al-Kalam is: to beat them with palm branches and shoes, to take them around tribes and to announce that this is the penalty of those who leave the guidance of the Qur'an and Sunnah and adopt 'Ilm al-Kalam' . He added: 'I have come to know of 'Ilm al-Kalam from its followers that which I could not imagine. So, it is easier for one to commit any sin except Shirk than to have 'Ilm al-Kalam' , i.e. if we compare between the fault of those who have 'Ilm al-Kalam and that of any other sinners, (except Shirk) we find the first is worst.
As for Sufism as seen nowadays and recent times, we think it is built on misguidance and innovation.
As for Ibn Taymiyah , he is Sheikh al-Islam , a wonder and exemplar in knowledge and understanding. He defended al-Sunnah and fought innovation.
Scholars and students were and are resorting to his books and works of Sharia knowledge. We pray to Allah to punish those who treat him as an enemy.
So, we thank you for your desire to spread truth to people. And we advise you be like a clever businessman who can offer his goods to achieve success and profit at all times, i.e. we ask you to choose the best manner convey the Da'wah of Islam with wisdom.
Allah Says (interpretation of meaning): {Invite (mankind, O Muhammad SAW) to the Way of your Lord (i.e. Islâm) with wisdom (i.e. with the Divine Inspiration and the Qur'ân) and fair preaching, …} [16: 125].
Thus, we advise to avoid speaking evil of the innovator you mentioned in your question, but you can explain the truth and demonstrate his misguidance and errors indirectly and with good proofs.
Here, we state that if you have patience, present the truth well and have a good intention, Allah will grant you success and light of truth will cover falsehood. This, for sure, is the result according to the Divine norms in this life.
Allah Says (interpretation of meaning): {… Then, as for the foam it passes away as scum upon the banks, while that which is for the good of mankind remains in the earth. …} [13: 17].
We pray to Allah to grant you success and guide you on the straight path.
Allah knows best.