It is very difficult to increase drain resistance. You have identified the likely routes, with the potential extra option of a Mentor spirit, drugs to ignore the pain, or 'ware to soak the damage.
All magicians use Willpower plus another
mental attribute appropriate to their tradition to
The base Drain Value for Spellcasting is listed in
the spell’s description (see p. 195.) For Summoning
and Binding spirits, the Drain Value is twice the
number of hits (not net hits) generated by the spirit
during the Opposed Test.
Each hit on the Drain Resistance Test reduces
the Drain Value by one. Any remaining Drain is suffered
by the magician. Drain is usually Stun damage,
though there are situations in which it can be
transformed into Physical damage (see the Sorcery
and Conjuring sections).
Therefore there are two routes to reducing drain, increasing attributes or cheesing the opposed test.
Now, Mentor spirit with spirits of man (according to this) may modify that test. (support here).
In the same area, they suggest better living through chemistry:
Buy Stim patches that let you ignore stun damage modifiers (No-Pain is even better)
From the same source, they note that you cannot use foci to resist drain:
Errata clarified that NO focus can help with drain.
Sacrifice does seem quite useful:
Sacrifice requires two Complex Actions completed consecutively. Taking any other action between the two required actions will negate the attempt. The initiate first performs a normal melee attack using the appropriate melee weapon skill. The target may attempt to parry, block, or dodge as normal. If the target is restrained or prone, appropriate melee modifiers apply (see p. 148, SR4). While any living creature can be used as a donor, the blood of sapient donors (metahumans and critters with the Sapience ability, such as sasquatches and dragons) is more potent. For each box of Physical Damage inflicted on a sapient donor, the
Drain in the subsequent action is reduced by 1. For non-sentient critters, the DV is reduced by 1 for every 3 boxes of damage (possibly less if the donor is significantly smaller than an average human). Spirits can never be donors, even if they are currently possessing a living body. An initiate may use himself as a donor, drawing on his own life force to reduce the Drain of his spells. A blood magician can inflict any desired level of Physical damage on himself. Sacrifice is the prerequisite for a number of advanced metamagic techniques.
This neatly reduces the utility of the "Bag of Rats" and causes some interesting moral problems. On the other hand, it also doesn't require the death of the target.
This is absolutely the way to go if you're the kind of shammy who summons a big expletive spirit at the beginning of the day and has it hang around. I get the sense that in-combat summoning is kind of inefficient and difficult in the first place.
This page suggests some 'ware alternatives:
I wanna make a hermetic mage with the restricted gear quality and Pain Editor cultured bioware for an extra die to my drain pool and to make it so he doesn't pass out unless he's dead. Is that cheesy? Of course, I know it would be up to my GM, but I am just wondering what the general consensus is on that combo. I was also thinking of getting Cerebral Booster, but I didn't that that was cheesy because it's pretty common.
But that is a massive sacrifice...