Greeting ya gits! It's time fer ya Education in da Orky ways!
As many of my collegues have mentioned you don't trust your boys enough, The
KFF is amazing at protecting the ork boys as very little in the game cuts the cover saves in which it provides and if something actually 'can' cut it...

Kill it....
The power of Orks isn't in our tanks, or our speed it's the ability to be the freaking incredible hulk in this game, yeah you can unload insane amounts of punishment my way... but I am coming to stomp you little man! Even though most people have gone over your list weaknesses with you I am going to go over again to make sure you know I harmonize on the important thing. I have NEVER had problems with any marine faction so allow me to help you either in here or private messages as I love fightin dem SPACE MARINES
YOUR LIST
Big Mek.
KFF - PERFECT not much needs to be changed here, the Big Mek is so frail that his best role is to actually avoid combat at all costs as to not be picked out in assualt any other upgrades are a waste other than maybe Bosspole since you have no upgrades in your boys group
20 Boyz. Not upgraded. - Unaccceptable! If the boys are going to footslog (meaning no transport) then I highly suggest you max them out Nothing stands against the might of 30 boyz without having the proper tools to take it on and Marines are by no means an exception. I run my troop choices for orks the same as almost any ork player i think 30 boys, Nob powerklaw, bosspole and Maaaaaaaaybe eavy armor, this is your best tool for dealing with things like Dreadnoughts and tanks as when nobs charge they are strength 9 with 4 attacks that cut armor saves... No one laughs at that the other 29 boyz are his body shield bringing him there to do that
10 Shoota Boyz. Eavy Armor - I like ard boyz from time to time but only with Grotsnik... even then 10 is simply not enough this is a trash unit since it has no nob or numbers I highly suggest dropping it.
5 Loota's - while... decent by themselves they only need to suffer 2 deaths to start freaking out about morale... and leadership 7 means this squad will quickly flee if you don't take it seriously
2 Looted Wagons. Boomgun/ram/armor plates/grots/2 rokkits/red paint job. - I have never used looted wagons with the boom gun as orks mostly cause in my mind I can't talk myself around 11 armor on front and sides being WAY to easy to destroy. I have used them as transports for units that needed to get somewhere but that was about it, I'd suggest you drop these but i do know the ordanice must be nice if the marines are on foot
5 Stormboyz. - I might anger the ork community by saying this but... I HATE STORMBOYZ ... I love them fluff wise but can't stand buy 1 ork for the price of 2 knowing they will be shot down before the rest of my army... it just makes me cringe everytime i see it! I do love the stormboyz models but they are a really ALL OR NOTHING type of unit either you take 60 of them or the enemy is going to have a hayday with that small unit. Simply put I can't imagine this squad surviving even one squad shooting at it.
3 Meganobz with Trukk. Usualy accompanied by my Warboss. - Not a bad selection here, the Meganobz and warboss can be very deadly... but if you are already using
KFF big mek... you really don't need both. Use the
KFF Big Mek as a launching pad for your trukk instead hide your trukk in cover advance the horde round 1 move the truk as far forward as it can while still being within range of the
KFF (it helps to have other nasty distractions like more trukks, wagons or deff koptas at this point). The Next round go flat out no mattter the cost and get
AS CLOSE
AS POSSIBLE if the enemy shoots it down which is very likely your nobz should be able to survive till round 3 when they declare the waaagh
3-6 Deffkopta's. : You are burning SO MANY points here for a 1,500 pts game. pts which could be used for boyz, upgrades and proper wargear. I love deff koptas though as does most the ork community but you need to use them right please read this...
http://www.dakkadakka.com/dakkaforum/posts/list/415487.page it will really help you with their tactics. Despite my previous posts the Deff Kopta is one of the only choices of the Ork Codex which THRIVES on being by itself or in small numbers. I suggest trying this out make 2 squads of single deff koptas give them
TL-rokkits and Buzzsaws and watch the magic as they round 1 assualt select targets like transports, scouts ect
10 Nobs. Painboy/cybork bodies/eavy armor/waagh banner. Sometimes Big coppa's. - While I commend you on the use of nobs I suggest against heavy armor if you already took cybork bodies as most people have ways to cut armor and will normally use it against your nob squad in the first place. While I suggest big choppas I also suggest the might
PK in this group as the ability to autokill that librarian that is giving you so much trouble trough one decent hit and a failed save is AMAZING!
Warbuggy. : can be useful in squads but if you only have one drop it for more points for the army.
The fundamentals of orks is to be able to outnumber your enemy immensely that anything they do won't influence the course of battle for you. That is done through many ways but the idea to most orks is simple game plans I'd say your primary fault here is wanting to have big nasty stompers like Meganobs but still have the tanks for a mek. here is my suggestion, you may want to look into the ork Tactic called "Kan Wall" it involves using a large number of walkers to shield your big mek and boys when i run it I normally have 11 walkers on the field 9 of which are Killa kans with grotzookas or rokkits depending on the circumstances of my enemy. It's a slower list but its one that grants 5+ or 4+ saves so very very often that it makes you feel like a Necron

. The best part to this list is you could go variant and keep the tanks as rear artillery and have alot of fun.
Right now i think that is your main thing, you need to realize that orks don't work well unless maxed, you need to choose a theme to your list either meks being your largest punchers or boyz. Warbosses are beastly but
IMO Big Mek Wins the day in our current Codex and I think you probably agree.
Now onto your enemy... He's a marine it means he prefers good statlines with QUALITY while your army prefers QUANTITY this is a perfect match up were no one has the real advantage normally speaking It seems like your friend/enemy has a good knowledge of how to fight you while you still don't though. One of the ork's biggest weaknesses
IMO is Flame templates as it really messes up
KFF and orky armor. You'll need to be aware that anytime you charge this guy he took those flamers knowing he could kill WAY MORE ORKS that way than through combat so make sure if you give him that advantage that you have a way of making him regret it.... like MORE BOYZ
The Dreadnoughts would be a real concern to me with your list because of the lack of Powerklaws on all but your most expensive models. If you get nobz with Power Klaws in your list your enemy won't be able to kill him he'll have to choose your boyz and then your nobz will tear it apart on a charge and move on.
Lol one of the funniest fators to me is that this marine actually outnumbers you right now or at least comes stupidly close to it, That is a math problem i can tell you is the main reason you find yourself in so much pain against him. Notice how his tact marines have MSSL? I bet all he does is spam rokkit frag shots on your 20 boy squad till they go away and if they don't he uses the flamer to finish them right?
The player's choice in terminators plays to your favor ... not sure the equipment as that is vital to know but even mighty termies can't pass a stupid amount of saves Normally i just charge them with no fear and wipe em since I have 30 boyz and a nob which reduces a 2+ armor to 5+ invul (noticing a theme here?)
The Scouts... honestly... should just be ignored unless they were deployed to distract you... IF I were a betting man I would say those snipers probably dealt with your lootas and then moved to things like stormboyz and then trying to pin advancing hordes or the Meganobz
So compare your list to your enemies and you can tell which clearly has more thought put into it.
Your army possessed 2
HQ's, 2 troops, 2 elites, 2 heavy support and 2 fast attack, ... while balanced at a glance the point is you barely fill these squads out... but then we look at your marine friend.
His army was 2
HQ's, 3 troops, 3 elites notice how he has no heavy support or fast attack? It's cause he fills it out with his MSSL's for heavy support and he realizes he can just shooting gallery you as an ork so no need for speed. As an ork... you need to make him regret the decision to have no speed. allow me to provide you with a list similar to yours which I think you should find produced better results... i'll try and keep the majority of elements that made sense to me.
1500 Pts - Codex: Orks Roster
1 Big Mek
1 Big Mek - Kustom Force Field; Choppa
60 Boyz
58 Boyz - Choppa & Slugga (Or Shoota, to preference but I'd suggest shootas)
2 Boyz Nob - Bosspole; Power Klaw
1 Deff Dread
1 Deff Dread - Skorcha; Armour Plates; Grot Rigger
[Unassigned: 1 Skorcha]
2 Deffkoptas
2 Deffkoptas - Buzzsaw; Twin Linked Rokkit Launcha
6 Killer Kans
6 Killer Kans - Grotzooka
10 Lootas
10 Lootas
1 Looted Wagon
1 Looted Wagon - Boom Gun; Armour Plates; Grot Rigger
3 Meganobz
3 Meganobz - Shoota/Skorcha Kombi-weapon
1 Trukk
1 Trukk - Armour Plates; Grot Rigger; Red Paint Job; Reinforced Ram
Total Roster Cost: 1500