I can quite imagine it to be very difficult.for a beginner. Also it is not possible in DSA5 to describe a unit's difficulty by a single number.
Because what is true when having a single fight doesn't simply add up to make a large fight. Two very weak units will smash a strong with ease and a mob of peasants will kill a smaller army.
Is that good?
I leave that to yourself but it's definitely not that bad. Often a strong fighter will lose against two weak ones - so it is realistic.
Enough talking! What shall I so?
At first I want to mention that - if your group agrees on it - you can just change your attackers' values. If they turn out to be too strong, why not change it?
I can understand that this looks like cheating and maybe it is, but doing that wisely will make it more interesting for everyone. I will not take this agreement as granted and the following points will not rely on cheating.
One after another
Often units are spread out in a wide area and the heroes will not know how many enemies will there be (one might think of a pub where there are more bandits downstairs or a forest where some are still hiding waiting for the perfect moment to shoot a hero down). If the bandits turn out to be to strong you can simply change your view of the scene and remove the waiting units (this will not change the players view since they never knew about the enemys in the first place). Also by sending single units you can get a feeling how hard it gets.
Going more than 1for1
They are heroes! So they shall fight more than one apponent, one might think and wrongly so.
Fighting more than one opponent will high likely lead to a lot of damag done to the hero. Remember: Defense is reduced by THREE for ever defense that has been done already in this round. Therefore getting two or even more opponents will eat up those health points like nothing else.
You can make this more reliable by sending 8 enemies against 6 heroes and let them switch targets from time to time. By that you can keep the whole group alife, not killing one by sheer force. You can storytell it by saying something like "One of your opponents shout for help and the large one comes running towards you. You are now facing two opponents". Then of course send them to the strongest hero or one who still has a lot of hp.
Last but not least
DSA is not DnD. Fights are ruff and you cannot expect to slash laughing through your enemy's forces. If you go and have a fight it will be an important one and there is a chance to die. Being aware of that we shall emphasize every battle even more. A single interesting fight is more interesting than boring slaughter.
Shields will stretch the fight without increasing its lethality largely.
'Two weapons' increases lethality largely while potentially acxelerating the fight.
Range combat (spells, bows ...) are lethal. Be very careful when using them. But crossbows have long reload time giving the heroes a fair chance after suffering the first wave of massive damage (more intense?)
Feints increase the damage a lot.
High armour slows the fight without increasing lethality largely.
Slow but strong, deadly enemies are great. Smart heroes can run away and shoot them from the distance or simply escape their death.
The environment can give heroes an advantage. Describe it briefly and give them hints how they can use it (better positions (+2/+2).is very nice), if they are new to the game.
Enemies can run away.
By killing their leader you can discurague the rest and might win a battle already lost.
Unexpected aid (use that rarely).
Dead enemies can have health potions with them (or similar).
Heroes can lose and losing doesn't have to be the end. They can be captured (use that rarely) or just robbed.
Always know the HP of your heroes (doesn't have to be exact).
(I'm not native English and play the game in German so I am sorry for mistakes and the possible use of different terms for describing in-game functions)
Enjoy playing it's great