The phrase "a difficult element to" is correct and usable in written English.
It can be used when discussing a challenging aspect or component of a situation, task, or concept.
Example: "Understanding the underlying principles of quantum mechanics is a difficult element to grasp for many students."
Alternatives: "a challenging aspect to" or "a tough component to".
The future-knowledge element is certainly a difficult element to bring into the already vastly complex timeline of the X-Men universe, but considering the very concept of the fourth-wall breaking hero seemed near impossible when first greenlit, it's clear this franchise is in the business of conquering cinematic challenges.
Despite some recent progress in constructing an on-chip optical isolator [62 64], this remains a difficult element to incorporate into the integrated photonics toolbox, and other techniques for ensuring unidirectional propagation are necessary.
As a further consequence of its high toxicity, mercury levels in biological tissues are usually very low, making this a difficult element to study.
Although some information can be used about past international migration, available from the Medical Registration Committee, this still remains a difficult element to estimate as it strongly fluctuates with the labour market conditions in other countries.
The controller is often the most difficult element to identify.
This is the most difficult element to develop in this architecture.
The world's financial downturn adds a difficult element as many of the programs to curb emissions require credit and financing.
The mens rea elements differ in whether they require a quid pro quo; a mere nexus is an easier element to prove; more difficult elements to prove include the intent to be influenced and inducement.
I should also note that wind strength is one of the more difficult elements to predict of a tropical storm.
Among the more basic, if politically difficult, elements to tempting people on to bikes is to reduce the flow (and speed) of motor traffic on smaller roads, those too minor to require a separated bike lane.
However, the development of high-resolution continuum source atomic absorption spectrometry (HR-CS AAS) instrumentation has allowed the analysis of challenging samples for non-metals as well as some difficult elements to determine by AAS, such as aluminum and phosphorus.
