Needs a way to deal with Siren' s call
=either multiple big creatures or psychic immune or no creatures
Needs a way to deal with psychic conditions and enchantments
=dispel and priestess's ability or use psychic immune or no creatures
Needs a way to remove armor
=dissolve and dispel
The only psychic immune creatures in the core set are skeletal sentries, and they're kind of frail. They die really fast without reassemble and there's only two of then in the core set. Which means you would need them to do as much damage as possible to the mage or to a big threat before they died. Problem is they can't gain elusive, so the only they can handle enemy guards using only core set is if you use knockdown or tanglevine/force hold. This is fine if there's only one enemy guards. If there's two or more you're going to have a hard time constantly reapplying knockdown, you don't have astral anchor to stop them teleporting out of your tanglevines, and you dont have enchanter's wardstones not arcane wards to protect your force holds. And before you suggest just including more copies, more copies takes more spellppints to include in your book and more actions to keep putting them back on after earlier copies get dispelled. Not to mention thr core set only has two copies of force hold anyway.
With the core set you can get up to 7 armor if you invest the spellppints. It only takes a rust and three acid balls to whittle it down to 0, and that's if you even have the actions to get that much armor. A battle forge in your corner is still very vulnerable to bring being destroyed early if you're not putting pressure on the enemy especially if you're facing druid or siren and you dont have any creatures to reliably protect it because only the sentries are psychic immune and you dont have intercepting creatures and you dont have mind shield. Against forcemaster you need unavoidable attacks or to restrain her so she gets lower def rolls. If you try restraining her with tanglevine shell teleport out because you dont have astral anchor nor a way to protect it if you did and shell also dispel your force holds easily enough for same reasons. The only way to get unavoidable attacks in core set x1 only is perfect strike or brogan bloodstone or flameblast. There are only two flameblasts in the core set and if you put on an elemental wand it will just get dissolved because flameblast is range 0-1. you try to use priestess's ability to deal with psychic conditions on brogan and to remove corrodes and weaks while using dissolve and dispel it get rid of enemy armor and a combination of evade on a mage wand and Mongoose agility to get past guards , and minor heals to keep him alive he while he attacks, that could possibly work as a tanky rush deck against everyone except the siren because siren' s call has no removal cost. Summoning multiple big creatures that aren't psychic immune could work if you could give them all elusive to get past guards. There are only two monoose agilities in the core set, so you would need one of your creatures to have innate elusive. That basically just means cervere. But cervere is expensive since he costs 15 mana. And if they dispel any of your Mongoose agility you could be in trouble. Not to mention you have no way to deal with swamp except to push or teleport both your threats and the enemy mage out of it. As soon as they have a swamp with astral anchor and an arcane ward, that's basically gg.
So how to stay viable while buying the minimal number of expansions?
That depends on what you're doing. If you want to rush with a buddy that isn't psychic immune, you need mind shields. That would mean forcemaster vs warlord.
If you want to do a solo rush with attack spells you need acid balls to deal with armor. Dissolve is range 0-1 and the majority of lasting mage armor is equipment which means if you try to get rid of their armor first you often won't be attacking until at least round 4.
That means druid vs necromancer.
If you want to tank the enemy out with dot, you need things that do dot the majority of which comes from enchantmentst. Ghoul rot and magebane are in the core set but they're often not enough. You either need to get force crush (forcemaster vs warlord) or you need astral anchor and stranglevine (druid vs necromancer and battlegrounds: domination). In either of these two cases you would need enchanter's wardstones to protect your enchantments, so you would need conquest of kumanjaro.
The list goes on. Core set x1 only spellbooks are no longer viable in competitive play. The last potentially viable core x1 only book after druid vs Necromancer was likely some kind of priestess book, and with Siren's ability such a book has become unviable.