The spell Shriek of Impending Shafts (Ars Magica p.136, InHe15) is a darn effective defense spell against wooden missiles. However it does not really neutralise the threat of an archer against others. So here are two approaches to better deal with archers.
Malevolent Gift of Roses
Muto Herbam 25, R: Voice, D: Diameter, T: Group
All items with wooden parts on the target are changed to roses. The spell is intended to be cast upon an enemy archer or soldier however it may also be used on any valid Group target.
(Base 3 to make one Herbam product into another, +2 Voice, +1 Diameter, +2 Group, +1 sub-parts of complex items)
The spell could also be used to ruin any item or thing which has wooden parts. Ladders, carts, and so on.
In the spell design I’ve opted to pay an extra magnitude to be able to affect the wood in component items, to affect a Group, and to do so for a fair duration in most fights – that is potentially overkill. The intent is to make the archer all but useless, and will probably make many parts of soldiers, workmen, and many other targets equally frustrated.
It would also be useful if scaled up for really large groups, however as it stands will affect a group of people in a fighting formation too as they form a valid target group. The limitation of this approach for arrows is that it is not plausible to affect all foes as there may be many of them and not in a group.
Ward Against Staves and Arrows
Rego Herbam 20, R: Personal, D: Diameter, T: Individual
Caster is protected against wooden weapons and missiles, which may not touch the caster.
(Base 15, +1 Diameter)
The ward could be changed to be R:Touch or an extended duration D:Sun for safety while travelling. These spells are part of the free new spells compendium for Ars Magica.
The Iron-Bound Tome 