I was hoping someone could give me some further explanation on when I'd want magic pen runes over ability power for ap casters? (Talking about magic pen for quints, seals, or glyphs)
Good ap ratios --> ap runes
Mediocre ap ratios --> magic pen runes
Anivia Mid GG
mp red ap/level blues ap quints
The above advice is good, but also keep in mind it's much much easier to buy and build ap than mpen.
Anivia - both 1:1 ratios on her Q and E
Thus you should take flat AP quints, but still keep your magic pen marks.
Whereas a caster like Brand has poorer ratios and a passive that scales only on magic pen
I usually run 2 flat AP 1 magic pen quint for Brand
AP is better for someone that uses ally-cast spells, so supports I guess (or not anymore, cause they have bad ratios on heals?)
Heals and shields are the example. They're not affected by MR. Seeing as Alistar can be built as tank, he shouldn't buy many AP items then (abyssal scepter maybe, cause of MR it gives).
I'm not a specialist in that matter though... I mostly use magic penetration for AP casters and the only tank I play is Amumu.
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