An extended example may be useful.
EDITED: There is debate over how Aegis affects incoming spells. I am using the clarification found in Hermetic Projects, which I cite below: if either the target of the spell or the caster is inside a hostile Aegis, there is a casting penalty of 1/2 the level of the Aegis.
Cain and Abel are magi who have declared Wizards War on each other. They have fixed arcane connections, have each other's signature from hate mail written over the years, and know each other's Hermetic names, but no other sympathetic connections. Both have Parma of 4, Penetration of 3, and are protected by an Aegis of 25.
Cain invents the following spell:
Spontaneous Combustion, CrIg 30
R: Arc, D: Mom, T: Ind
The target takes +15 fire damage.
Base 10, +4 Arc
Presuming Abel has Ignem 10, Cain needs a 52 to affect Abel with this spell: 52 -12 for Abel's Aegis = 40 - 30 spell level = 10 penetration + (3 x 7 = 21) = 31 penetration total, just enough to overcome Abel's Ignem resistance of 20 Parma + 10 Ignem. You can decide for yourself if a 52 is moderate or hard, but it's certainly not easy. For magi of this level, inventing a level 30 spell probably took a couple of seasons.
Abel, on the other side of the Tribunal, is instead using Intangible Tunnel. He invents a level 15 version of the spell, substituting Diameter duration for Concentration. He also develops the following spell:
Pull My Finger, MuAu(Co)
R: Touch, D: Diameter, T: Part
This effect transforms the air entering the target's lungs into a noxious sulfur vapor that causes +3 damage each round for 20 rounds.
(Base 3, +1 Touch, +1 Diameter, +1 Part, +1 Corpus requisite)
Presuming that Cain has arts of 10, Abel needs a 37 to succeed with first the Intangible Tunnel and then Pull My Finger: 37 a 12 for Cain's Aegis = 25 - 15 for spell level = 10 penetration + 21 = 31 penetration total. A 37 is still not easy, but is certainly far easier than a 52. Abel had to invent two spells instead of one, but they were both easy to invent. The strategy is not without risk; if Cain has a way to percieve an Intangible Tunnel (but only if), he can attack back through it. Abel could develop additional spells, such as a a PeCo 15 to inflict Medium Wounds on Cain, or a PeVi spell to close his own Tunnel, and so on and on, in the way of Hermetic magi through the centuries.