Never count out the giant frogs entirely, they can be really fun monsters. The un-fun bit about them is the stunning. The fact that they swallow people for ongoing damage and can then run away with them is hilarious and completely changes the tactics in a combat.
In my game, the players were winning the fight, then a frog popped out of hiding in the mud, swallowed a caster whole, then ran away up stairs in the tower they were fighting in and tried to hide. The caster had little chance of getting out himself, so other group members had to break away from the fight and chase down the frog quickly before he got digested.
To maximise froggy fun, I suggest the following:
- Have lots of mud around so that the players can be ambushed, or at least keep wondering if they are going to be ambushed.
- Make the frogs run away, preferably over lots of difficult terrain mud once they have grabbed a victim.
- Frogs try to hide and enjoy their meal. It makes for a good scene when the PCs follow the frog around a corner, then can't see it. A perception roll reveals their companions feet sticking out of the edge of a mud pond.
- The person swallowed is restrained, not stunned.
- Make it very very hard to get out of the frog by yourself, thus your friends need to help, but your not bored waiting for them to do so.
- People can make a grab attack to try to pull their companion out, giving them another save.
- Hitting the frog for 16ish damage or more makes them vomit up their meal.
- All of this going on while a bunch of weaker bullywugs are trying to fight the party is a lot of fun.
Sorry for the lengthy post that is only semi-relevant to the topic. Hope it inspires someone to make a fun fight though.