It's so hard to watch you snatch defeat from the jaws of victory.
>the boost thing
If you have a slot available, you may boost any lesser tier spell to that slot. Remember that Yulya has 2 2nd-tier slots and 8MP. There are no slots for 1st-tier or Cantrips, they are limited only by MP available.
You may, using a focus, boost above to a slot you are not otherwise capable of casting, this would be any spell you are capable of casting.
It would automatically use the highest tier slot you have available.
It would cost the level of the tier, so for Yulya, boosting to tier 3 would cost a tier 2 spell and a focus with 3MP.
However, unpredictable results may occur because you are not proficient in that tier.
You may however, use a focus to cast a spell to a tier you can cast without a slot available, such as right now, Yulya could cast another 2nd tier spell using the focus with 1MP left over, that 1MP would be considered extra energy and will not be wasted. However, even though you could reasonably cast a 2nd tier spell, there is a chance that it can go wrong because you are effectively overloaded because there are no more 2nd tier slots.
It would cost no MP at this point.
You cannot use a focus to cast a spell as a use action, but you can use a scroll in this way which is another way to cast more than one spell per turn, but you only get one use action.
Having 3MP left, Yulya can cast one spell, she couldn't cast a tier 3 boost at this point because the focus would need 4MP, 1 for the up-boost to tier 2 and 3 to boost to tier 3, but if you cast Sacred Flame and said you wanted to use the Focus to boost it, there is a small chance you will get more than you expected in a good way and in a bad way. I consider this a very fun mechanic and potentially amusing.
She has one use left on her ring of quick casting, so she could cast twice with or without the focus. She could also attack him directly with her mace or even attempt to grapple him.
Remember that filling a focus is a simple action that consumes your mana. You can freely add MP to a focus. There is a feat that allows you to remove it like a battery to replenish your own MP which has the advantage of not having to expend the whole focus at once.
If you are completely out of MP, you can still use a focus to cast. Focus in this case is used to describe a gem with mana stored inside.