Naiad heartpiercers - bring two (or even three) regiments and you’ll have a very strong shooting base that will melt low-defense units like zombies in a turn or two.
Alternatively, bringing a regiment of earth elementals against thrash armies is hugely fun. The amount of turns a me5+/no CS unitis going to need to break through that defense 6+ is insane. If your battle plan involves shooting (greater fire elementals + scorchwings) from behind this batle line, this works fine. To add insult to injury, a cheap heal on a druid can heal that few points of damage that does come through.
You could also opt for forest shamblers. They are slightly less durable (due to def 5+ instead of 6+) but they are cheaper and do scout.
other option: bringing a pair of surge casters to do surge-tricks. Finding a flank with a greater fire or greater air is very rewarding. (though not the most easy play, so maybe this is not optimal)
Finally: fire elementals have more offensive power due to CS(2) and vicious. The greater fire is an absolute monster with CS (3), melee 3+ and vicious. Find a flank with one of these with a surge trick on your 50x50 base and you’re looking at 12-ish wounds against most units.