It's not even close to difficult! Perhaps you are still using the R from your native language, and this is causing problems.  Once you learn an English R - it's an entirely different sound - the problem will disappear.

I wonder if you are trying to give all three syllables equal weight? This is one of the problems which Spanish speakers have with multi-syllable words in English. Many Spanish speakers concentrate on saying the individual letters and sounds, but ignore this key aspect of English pronunciation.

The three syllables of 'murderer' are not XXX. They are Xxx - one stressed followed by two unstressed. Remember that it is only the first syllable ('mur') which is stressed. The other two are very weak and unaccented, and the vowel sounds are  /ə/ - the effortless schwa sound. So in British English, it would simply be /mɜː də rə/ or 'murd-uh-ruh'.  

Or if you really want a short cut, you could even say 'murd-rah'. The middle syllable is so weak that it is barely there in some pronunciations. If a GB English speaker said 'murd-rah', this would be understood.

And I agree with Oliver that you might find the American pronunciation a little harder, as there are more 'r' sounds there to get tangled up in! If you don't mind pronouncing this the British way, you might have fewer problems.

Now for confession time. I have trouble saying 'jewellery'. There are two possible spellings for this word and several different pronunciations, but I don't feel any of them sound right when I say them.

I don't find it difficult. But we use a very different "r" sound.

Notice that a dictionary shows the pronunciation of the word as 

mûr'-dər-ər

That upside-down <em>e</em> is the <em>schwa </em>and we use it all the time in English. It is very close to leaving the vowel out and just speaking two consonants one after the other. I don't know if that makes it easier to pronounce. 

Some words that must be difficult to pronounce, because they are often mispronounced are:

<em>ophthalmologist</em>--the <em>phth</em> combination is difficult and unexpected in every words in which it occurs

Maybe two r's <em>are</em> difficult, because two words that often are mispronounced are:

<em>library--</em>the first "r" is often omitted

<em>February--</em>often mispronounced "Febuary"

<em>asked</em>--difficult, often mispronounced "ast."

<em>didn't</em>--doesn't seem hard to me, but I often hear people <em>ditən. </em>I guess that's what happens if you switch the order of the <em>t</em> and the <em>n</em>.

Murderer is not that bad, for one you actually pronounce it the way it is spelt! I guess that is an issue sometimes with English words. Sometimes I find that non-native speakers say the individual syllables correctly but put the stress on the wrong part of the word so it sounds a strange to a native speaker. My Hungarian friend once tried to say 'viscosity' and got in an awful mess with it, he couldn't get the stress right for a long time.

For me, in Spanish I don't really have many problems pronouncing words individually, but sometimes it can be problematic when I'm talking quickly. Mainly with stressing the wrong part of the word. A word I had problems with recently was 'desaparece'. It was at the end of a sentence and I couldn't get the stress right when there were words in front of it when talking at normal speed.

That is interesting to know....I would assume that pronouncing that word is harder in an American English accent than a British or Australian one. The words I notice people (especially native speakers!) mispronouncing the most are library, supposedly, etcetera, nuclear, and espresso.