I enjoy figuring things out by myself (I do use the wiki now though) so I haven't been watching how others play, but I did hear from somewhere that "Enlarge" on an Owlbear druid makes jumping on enemies from a height very powerful. I'm just starting out an honour mode run, where later I'm planning to
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Get 3 druids jumping on the Adamantine Golem from high up, which should in theory be enough to kill it in one round. I did that with a single druid on tactician difficulty and it did 150 damage
Early game I've been abusing Warlocks with the Darkness spell and Devil's Sight. It's a really good combo where most archers will just give up and stand passively, while melee enemies have to fight with disadvantage and I with advantage. The Shadow Blade spell is also very strong on a Warlock, dealing more damage than even two-handed weapons (2d8 and at higher level 4d8) .
Another mechanic I'm gonna abuse is Warding Bond, which I didn't initially like until I found out you can have an expendable character cast it on a party member then send them back to camp, and the buff will still be in effect. It's a bit evil from an RP perspective, but hey I'm playing as Astarion this time around so moral flexibility is in character.