What are you suggesting exactly? You have to allocate attacks ahead of time. Allocating all of the attacks against the steg can work, but the bigger issue is will he let you charge into the steg (especially into its flank)? That thing has a movement of 6, so it's not exactly a slow moving guy. Your knights have to not be tied up in combat with another R&F troop (which will have steadfast and lock down your rather expensive unit). Personally, I would use skinks with kroxigors in there. Skinks are 5 points a pop so the str 7 is wasted on the mediocre toughness of skinks while the krox's are swinging back with plenty of str 6 attacks, but not all LM have that unit

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The str 4 attacks against T6 and armor 3+ won't do a thing (barring really good luck).
Your knights also have to survive the barrage of flaming salamanders (which are -3 armor save) and magic of the slann. If the slann is more of the buffing type, then it would be likely to expect the steg to be buffed (high T will ruin your str 7 as would regen) or else why would he not mind the fact it's in charging range of the knights? It has 5 wounds and if you fail to take them all down in one go, your knights are in some serious trouble, 3 str 6 attacks plus str 6 thunderstomp will shred through the 5 of them like nothing, plus the d6 str 5 no armor save hits from burning alignment during the next magic phase.