Hena, if you check the original post, you will find that I also went through the scenario of the Chosen going second. Even if they sustained on the Chosen (which is probably their strongest means of attacking the enemy) they are very unlikely to be able to destroy the Chosen outright (unless the BL player is using my dice of course

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Usually in a straight power comparison like this, the results are fairly similar and scoring the same number of hits using the 'preferred' or strongest means of attack. But here, we have two formations that are equivalent in DC, where the results are far from equal.
My contention is that this imbalance is at least in part because of the power of the daemons being summoned, which significantly boost the offensive strength of this (or any) formation.
And yes in reality there are options open to either player. In reality, much depends upon the strategic rating of the races involved. Happily in this instance we can project the example onto the two scenarios of BL Vs Space marines and BL Vs IG.
- Against SM, the Chosen are likely to go second and the Reaver will walk away (unless it is surrounded by teleporting Chosen of course

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- Against the IG, the Reaver is likely to be toast (assuming the BL player has used some other means to clear a path to it.

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In both cases, the Chosen
will survive the first activations and can attempt to camp out on the opponents Blitz; and will certainly draw the attention of the enemy - which is why you fill the formation up with Fearless DP and Obliterators, so that when it eventually gets broken it can still camp in place and spawn more Daemons.
Indeed, an equally nasty idea would be to upgrade a Chosen(4) formation with three Obliterators and pact, and teleport into the enemy army. Costing 510 and summoning an average of '4' Daemons this is going to hurt, and may arguably be more survivable than the 565 Chosen +trimings above.