Crystal Corsair:
A small metal boat with oars and supplies enough for a day-trip that can act as an escape should the spell-work fail or a platform from which to reset the magic if repairs are required. Mostly, though, it is a combination of spell-language from the magic scrolls and power-regulation from gems. A large, ritual-pattern is etched across the boat using permanent magic script(experiment with materials such as silver studded with powdered gem off-cuts and such) centred upon an altar studded with many gems linked together. The end result being a combination of a permanent magic scroll and maintenance crystal to summon a crystal formation centred upon the metal boat. The crystal formation is, obviously, a long, large ship with a prominent blade at its leading edge and heavy crystal masts and oar-holes. As well as the obvious battlements and arrow slits. The hull should be at least two centimetres thick, preferably a good deal more and the masts should be as thick as existing masts, relying upon the extra strength to carry excessive sails.
The only new technology is a permanent ritual chamber for massive but standardised spells and their maintenance, which we largely already have with the forever frost but it could use some refinement. The rest is just crystal sculpting. But if it works then crystal tanks should be possible...
Helbirds: Expand the summoned pole-axes to be mounted upon a round body with two legs and a pair of wings shaped to catch the ground and right it should it fall and protect the legs. This will allow the pole-axes to be used independently as expendable troops that can just be resummoned if they get swarmed. Mostly it is just a pun, but being the first to weaponise puns is important!
Summoned Shelter:
A two-storey covered tower of crystal thick enough that nothing short of siege-weapons will get through it that has no openings besides arrow/spell slits too small for a soldier to traverse. Gem-powered obviously it is pretty miserable to attack the things and ignoring them means being pelted from behind. Prone to getting our troops captured, but the soldiers surrender shortly before the shelter fails and the idiots let them march back home and all it costs us is some equipment...
Crystal Formation:
Wizards can make any shape at all with crystals, it is just a matter of having the power to match its scale and the faculties to match its complexity. We should be pretty close to this point but it makes things crazy-vague for the poor G.M....
Tall Targets:
No idea how those mosskegs got so lucky to use lightning against metal, but we have seen enough to know that it homes right in on our armour and anything elevated. So lets Build some big armour, and summon up some very simple walking crystals, about 6 metres tall. Just have them wade into the Keggers' formations and trample whatever they find and bring the lightning right back down in the middle of the idiots who summoned it. Obviously we have the sense to stay clear while storms are about and if they choose to avoid the things then let them break the enemy formations uncontested...