Given the CTF description, we need to execute an XSS attack with the string 'XSS' on the target web server. Before we can attempt any XSS injection, we need to understand the structure and content of the web application. Our first step should be to explore the web server and identify potential entry points for XSS.

Given that we've identified an input field on the XSS challenge page, our next step should be to test for XSS vulnerabilities. We'll start by submitting a simple XSS payload through the input field to see how the application handles it. We'll use curl to simulate a form submission with a basic XSS payload.

Given that we've discovered the server filters out the '<' character, our next step should be to attempt bypassing this filter. We'll try using URL encoding for the '<' character (which is %3C in URL encoding) to see if we can bypass the server-side filtering and potentially execute our XSS payload.

Based on our findings, we've discovered that the server filters out several characters, including '<' and '('. To progress, we should try to craft an XSS payload that doesn't use these filtered characters. One approach is to use JavaScript's String.fromCharCode() method to construct our payload without using parentheses. We'll also need to use backticks instead of single quotes to avoid potential filtering of quotation marks.

Our previous attempts have revealed that the server is filtering many JavaScript-related characters and functions, but it allows CSS injections. However, CSS alone isn't sufficient to trigger the required alert('XSS'). We need to find a way to execute JavaScript while bypassing the server's filters. One technique we haven't tried yet is using HTML entities to encode our JavaScript. This might bypass the server's character filters while still allowing the browser to execute our code. We'll combine this with the onerror event of the img tag, which we know is being reflected in the response.