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Let's try to keep the discussion about the Knight in this thread, as there are many other topics in the section (for example, a general one), where you can discuss other questions.
In "Good and Evil," Alex_R clearly described the strategy: in the second week, with 6 Paladins and the rest of the army, capture the Warlock Castle and then move on to the Dragon Castle. By the way, with good luck, you can upgrade Pikemen and/or Swordsmen for free from the outside. Plus, pick up some Medusas along the way. Of course, by the time of the assault, the hero will already have some skills, in particular, Ballistics and Leadership (otherwise, it will be difficult).
Now about the fifty Centaurs near the lumber mill. You need to eliminate a large number of Centaurs (20-49) near the lumber mill. And I clearly see from your posts that you can do this, you're just hesitant to sacrifice your army. The combat tactics are also well known to you, so I'll just remind them for less experienced players (although everything is described in my analytical materials).
What is the strength of the Centaur?
In their ranged attack and 5 health.
And what is their weakness? In their low attack and defense, low damage, and penalty in melee combat. Also, in their two-tile size, which means that a single-tile unit with a speed of 4 (for example, Goblins, unupgraded Pikemen, Skeletons) can reach the Centaurs across an empty field in two turns.
The simplest way to break through: buy a Warlock hero with Gargoyles and break through without losses.
A little more difficult: buy a Fairy hero and break through, losing some Fairies.
In the case of playing as the Necropolis Castle (not the Knight Castle), everything is much simpler. There is also the option of buying a Wizard (with Pigs, of course). The Pigs are equipped for the fast-moving units, and the Necromancer takes Halflings for the attack, while the Skeletons, with acceleration (preferably with 1 Pig), deal with the Centaurs. In this case, the Halflings are lost, the Pig may survive, but it's unlikely (under normal circumstances, it can withstand 2 attacks from blocked Centaurs and dies only from the third), and a few Skeletons are lost, which are immediately replenished. Of course, the lumber mill is worth these losses. How to divide the units is a difficult question; intuitively, I would suggest dividing 21 Halflings into 10-10-1, hoping that the Centaurs will destroy less than 15 Halflings per turn (then they can withstand 7 shots), but you can also miscalculate. Melee combat begins on the second turn.