I think standard magic is too unreliable to really be a powergaming force, there is too much risk of a miscast and it costs a LOT of points. Slann and 3 engines takes you to somewhere around 1600 points? That is huge considering how soft a priest or even the Slann can be. To be a magic power army you really need insane spells and wizards of value in combat as well as at range, the old Tzeench list was a good example, or a VC list can have an insane amount of very easy to cast very good spells.
Although a Slann and 3 engines is a really really powerful amount of magic.... It would be interesting to see if any of the 8 lores could really bring enough power for the list to dominate.
I would ignore a lord altogether, though an old blood on carnosaur with blade of realities would be hard to look past, many many things that he can deal with easily.
Terradons for marchblocking, yes, and warmachine hunting since our biggest threat would be things that could kill stegs easily. But do not discount the power of d3 s4 rocks from each terradon if you have a decent sized unit; 6 terradons will average around 12 hits which can be dropped on basically any unit from turn 2 at the latest with a very wide range of potential.
TBH though I don't think LM are really a powergaming cheesy army, max stegs are scary to most but you need 2 to ensure you take a unit with their static combat res and there are some armies that can deal with them.
