The progress Rui made was unsteady, but it was definitely there. He made remarkably quick progress at certain stages while stalling for longer than he would have liked at other stages.
However, he definitely made progress, he was sure that.
While he was mastering his technique, he came up with his modus operandi for how he was going to take down his target. The most important part of the execution of the assassinations was ensuring that no one ever caught on to the cause of death.
The medical technology of Gaia was quite impressive, however, that was largely due to the fact that they had magical substances that could accomplish magical feats with relatively little ingenuity and rigorous science from the people who used them.
Based on everything he had seen, he was not only certain that they did not possess the ability to determine the cause and the time of death but also that there were no magical substances that could help them out this time.