As Airact stated, if they are flinching from going for board just due to Boris, take every bit of advantage you can from it. This isn't good play on their part, as Boris himself can only account for a single, generally easily replaceable ally, once in every 4-5 turns, PotL/Shadowspawn/etc not considered.
They should be trying to bait use of Boris' ability onto a persistent 3cc critter, not to mention Crippling Blows and Retreats, while ramping up to higher cc allies that are better targets for CB/Retreat. Even playing a reactive style deck that is aiming for the late win, contesting as much as you can as often as you can is typically the best play, weeding out counters and dividing damage that would be going directly for the Hero with allies that can afford to be expended.
It is also possible you've just run into decks not running lots of early drops, like only using 3cc and up allies, but regardless of this they should be dropping something on at least T3 unless they've just been very unlucky with hand/draw.
Cheers,
-Harakhte
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