To cross the bridge in Mount Gemma, you must defeat Adventurer David's miscrits. Here's some help on that.
The order of his Miscrits (All level 25) are:
1. Tigristrike (Statikat Evolution 3 - Lightning)
2. Duocornius (Equestrion Evolution 3 - Wind)
3. Moledozer (Drilldent Evolution 3 - Earth)
4. Pharaohette (Mumbah Evolution 3 - Earth)
To some people, Moledozer quite hard to beat because it uses Sandstorm and then slowly bash your Miscrit to death (sometimes, unless your miscrit's Physical Defence is Max) but Pharaohette is probably the hardest to beat. It constantly uses his healing ability. So unless you can do over 20+ damage to it on every turn (even with it boosting up its defenses by +11), then maybe you should wait till your Miscrits are leveled up more. P.S. Potions can be a lifesaver!
So in order to be most effective in battle, arrange your Miscrits like this before attempting to attack him:
1) Earth ----Defeats Lightning
2) Lightning ----Defeats Wind
3) Wind----Defeats Earth
Any Level 30 Miscrits
The order is good because unlike real live opponents, Adventurer David can't change his Miscrits. Hope this bit of info helps ya. Good hunting!