Uhm - ok.
I'm the OL of my group too and i can understand ur frustration and anger very well. in our first campaign the heroes just melted through Gravorn turn 1, leaving my dull and angry.
Things i realized:
1) if i aim for it i can win every encounter flat. but that doesnt make the heroes feel good, nor does it make me feel good, since i'm 1 player happy vs 4 players being angry and frustrated. after all, you are playing together on an epic campaign. i found it to be good if the games were balanced. hell, my heroes even then started to cry over me winning 50% of the encounters accusing me of powergaming, ruleabusing and harcore plays, where they did nothing less. but thats the role of the ol afterall i guess, to give the players a well balanced game experience.
2) if u let your heroes grow strong soon, they will faceroll over your quests, try let them win 1-2 scenarios, or 1 scenario and 1 encounter. if they win too often and get too much gold they will be no match for your monsters too soon.
3) always keep in mind what your quest objective is. its not about killing heroes or spawning monsters. in most cases its something u mutch better achieve by beein stealthy, quick or disruptive. try to hold down the heroes (ettins?) and slow them down, while planning to kill all that npcs, or buying time for spligg, or turning around all the guests. ALWAYS keep in mind EVERY turn what ur abjective is and work for that.
also keep in mind, that some cards are much more needed in the later encounter. a dash could save ur spliggs escape in (a fat goblin), so try keep it up your hand. also a frenzy enables him to move AND kick a hero out of his way with knockback.
4) try to get an idea of what u get from killing heroes. if it is 2 ettin attacks perfect. but if u have to dash, frenzy dark might and dark fortune 1 ettin to get 1 kill, u just traded 4 useful cards for 1 and a dead hero, which is a rly bad trade. again: keep in mind what ur objective is
5) heroes have the abilities to melt through most of ur monsters turn 1 (which is frustrating, but important, if they couldnt the OL would be far too overpowered). try to postion them smart enough, so that 1 or 2 monsters see ur turn and then use them smartly again. try to buy time for ur main objective, or lay some waste on the heroes. a hit and run tactic is in most cases way more clever then to run for the squishies to deal the most damage.
6) try to seperate heroes. most heroabilities rely on the heroes standing adjacent or within a range of 3. a nice ettin throw or a merriod immobilize fucks up their whole strategy. also a hero, which is parted from his group is a much more easy target 2 kill with ur respawns, traps or other stuff.
7) work with conditions. fatigue is a dire ressource which heroes will give up reluctantly, a low attributed warrior will suffer fatigue damage every turn if he is diseased, an immobilized melee can do nothing but rest, a poisened low powered mage will suffer 1 damage every turn (they **** up power rolls so often its hilarious). words of misery punches up ur damage enormusly, keep it in the back of ur hand and then, when the heroes are surrounded by enemies lay waste on them. if they suddenly start their turn full fatigued they will look quite shocked.
8) get the conversion kit. it will give u much more ressources (in form of monsters), which will give u dire needed viability. u can pick monsters which are (for the quest settings) much more appropriate and will turn the favours in your hands (and giants are just off the hook awesome).
8.2) use sentinel hybrids, they are above average, deal solid damage and are quite tanky. red one can even affect 2-3 targest (fire breath).
again: use disruption, move your heroes apart, use stealthy hit and run actions. and dont **** up your heroes, they will loose interest if u win every encounter