Ummmmm, if you're not getting a +10 in that situation there's something seriously wrong. A level 5 fighter with 18 STR, a +3 prof weapon, and Expertise is ALREADY at +9, and almost surely has a +1 weapon for +10 BEFORE any situational bonuses. The same goes for an 18 DEX rogue. Even assuming you DON'T take Expertise (you don't really need it at this point) you're talking a baseline +9 for fighters and rogues, and a baseline +8 for other weapon users with EITHER a +3 weapon OR Expertise. So throw in a -2 to enemy defenses and it would be pretty hard to NOT have a +10 with most any weapon using character. Toss in CA from flanking as a possibility and you're way there.