Which tones are hardest, why, and where to start.
A dedicated disyllabic-tone perception study exists: 12 beginner Tanzanian learners with Swahili as L1, 84 disyllabic tone combinations, run in E-prime. Its specific findings (which tones are hardest) could not be obtained, so no direction is given here.
Source: 坦桑尼亚初级汉语学习者双字调感知研究. 万方学位论文 Y3145251 (2016)
Group averages tell you where to look, not what you actually do. Two speakers of the same first language can differ a lot. The only way to know is to record yourself and compare.
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