Mechazor:
Mechazor is easier to deal with now than ever before. Almost all decks are running something in the main deck that counters it (Sunbloom, Eternity Painter, Sandswirl Reader, Chromatic Cold) or can outpace it (Aggro Abyssian, Starhorn burn). The only deck that somewhat struggles against Mech is Midranged Vaath, and even then the deck has enough value to overwhelm Mech decks. While Mech is still pretty boring and uninteractive, it is much easier to deal with than before.
Vanar Arcanyst:
This deck did get a few counters. Magesworn and the spell sentinels discourage their spell spam and Eternity Painter and Aperion’s Claim act as mass hard removal. Otherwise, the deck’s old weaknesses to Aggro Starhorn still remain. They even got a new Aggro deck to worry about with Aggro Cassyva.
Spell sentinels:
If Songhai plays a sentinel, it’s almost always Oni. Unless you absolutely have to, avoid playing spells until you deal with it. Alternatively, you can fill your opponent’s hand with useless spells before playing big ones. BBS, Obelysk spells, and card draw are generally okay to play into Oni.
Basilisk is almost always the Vanar sentinel. Again, use spells sparingly unless the upside for you outweighs downside of giving them the spell. Positioning this minion is also pretty tough since playing it too close the action makes it easier to remove before flipping. This allows you to run away and ignor it until you come up with an answer. Vanar generally runs 3 hearthsisters for mobility, so be weary when ignoring this minion
Lavaslasher:
This card deserves no in depth analysis. The fact that it is broken has been pointed out for months now.