I did a strategy for this earrlier this year, here is a nice link for you
http://ich.18.forumer.com/index.php?showtopic=1042
Ive found that the easiets way to beat the comp in these mission is to build cheap semi tough units that you can just spam like crazy instead of a fewer strong units.
as for defending your base, ive never need more than 4 AA and 6 SB towers for defense.
you could go Hippo/chamelons BUT.. i say that crocodile/chameleons (just cham head) makes for a much much better alternative.
Oh and at least once I mad a HUGE army (like 50ish or so) of lobster dragonflies. they have no HP to speak of but DAMn do they deal dmage to buildings fast. a couple waves of those and velikas base si GONE!